Logistics domain fundamentals

An order is a commercial intent: someone has agreed to buy or move a set of goods, and it carries prices, terms and a promise. A shipment is one physical movement of goods from one place to another under one transport arrangement. A consignment is the contractual unit handed to a carrier — one sender, one receiver, one set of documents — which is what a waybill actually covers. One order can become five shipments, and one shipment can carry consignments for forty customers. Systems that treat them as the same row cannot express a partial despatch, cannot split a backorder, and cannot answer the two questions every logistics business asks daily: what did we promise, and where is it now.

The entities nest, and each level exists because something in the business changes at that level.

order OS-99120                       commercial promise, 3 lines
 |
 +- shipment SHP-A  Leeds -> Bristol   one movement, one carrier, one waybill
 |   |
 |   +- handling unit SSCC ...0001     a pallet, what a forklift moves
 |   |   +- carton C-1   GTIN 5012345000015  x 24 EA
 |   |   +- carton C-2   GTIN 5012345000015  x 24 EA
 |   |
 |   +- handling unit SSCC ...0002     a second pallet on the same shipment
 |       +- carton C-3   GTIN 5012345000022  x 6 EA
 |
 +- shipment SHP-B  Leeds -> Bristol   line 3 was out of stock, ships Thursday
     +- handling unit SSCC ...0003
         +- carton C-4   GTIN 5012345000039  x 12 EA

Read the levels by what changes at each. The order level is where money and the customer promise live. The shipment level is where the carrier, the cost and the transit time live. The handling unit level is where scanning, custody and damage live, because a scan is always against a physical thing somebody can put their hands on. The item level is where inventory and picking live.

The classic conflation is putting the tracking number on the order. It works until the first split despatch, at which point the customer has two parcels and one tracking number, the second parcel has no identity in your system, and support cannot tell them where it is. The mirror error is putting quantity on the handling unit rather than on its contents, which makes a partial short-shipment unrepresentable — you can say the pallet arrived but not that it arrived with twenty-two cartons instead of twenty-four.

The third conflation is treating the handling unit as disposable. Packaging nests: cartons onto a pallet, pallets into a container, and a cross-dock may break a pallet down and rebuild it into three others. If the handling unit has no stable identity across that rebuild, the scan history breaks at exactly the point where goods most often go missing, and you lose the ability to say which custody transfer the loss happened in.

What all three failures have in common is that they surface in operations, not in development, because a happy-path order has exactly one of everything.

A load is a carrier-side unit: the set of consignments assigned to one vehicle for one journey, with a sequence of stops. A shipment is a customer-side unit: goods moving from A to B. The distinction matters because they have different owners and different optimisation objectives. The shipper wants each shipment fast and cheap; the carrier wants each load full and each route short, and those goals conflict directly. Load planning is where the conflict is resolved, which is why the same physical parcel appears in a shipment table keyed by customer and a load table keyed by vehicle and departure time, joined by a stop.

GS1 identifiers look like arbitrary numbers and are not — each field is allocated, and knowing the structure explains why they cannot be generated casually.

GTIN-13  identifies a product, not an instance
  5 012345 00001 5
  |  |      |    |
  |  |      |    +- check digit, modulo 10
  |  |      +------ item reference, assigned by the brand owner
  |  +------------- company prefix, licensed from GS1
  +---------------- GS1 prefix indicating the issuing member organisation

SSCC-18  identifies one logical handling unit, uniquely, worldwide, once
  3 5012345 000000001 6
  | |       |         |
  | |       |         +- check digit
  | |       +----------- serial reference, the sender must never reuse it
  | +------------------- company prefix, same licensed block
  +--------------------- extension digit, sender's own use

GS1-128 barcode on the pallet label, application identifiers in brackets
  (00)350123450000000016 (02)05012345000015 (37)0048 (10)LOT7741 (15)261130
   00 = SSCC            02 = contained GTIN   37 = count  10 = lot  15 = best before

The engineering consequence of the GTIN is that it identifies a kind of thing. Two cartons of the same product share a GTIN, so a GTIN can never be a primary key for a physical unit, and a scan of a GTIN alone tells you what something is and nothing about which one it is.

The SSCC is the opposite: it identifies one handling unit and must be globally unique and never reused. That single constraint means serial allocation cannot be a random number or a database identity that resets on a restore, and it means handling-unit labels are a resource with a lifecycle — allocated, printed, applied, consumed, and eventually retired after the reuse embargo.

The application identifiers are where integration work actually lives. A pallet label carries the SSCC, the contained GTIN, a count, a lot number and a date, and a receiving scan can therefore create stock, attach traceability and check shelf life in one action. Sites that print their own non-GS1 labels lose all of that, which is why their receiving is manual and their recalls are spreadsheets.

The interview point is that these standards exist so two companies who have never spoken can read each other's pallets. Reinventing them is not a shortcut; it is opting out of everyone else's automation.

Because the ratio of stops to volume inverts along the chain. The first mile collects from a small number of senders in bulk, so cost per unit is low and the variable is scheduling. The middle mile is line haul between sorting facilities: high volume, few stops, and cost driven by vehicle fill and fuel, so it rewards consolidation above all. The last mile is one drop per customer, so the driver spends most of the day stopping, parking, walking and knocking — labour dominates and volume barely helps. That is why the last mile is routinely half the total cost of a parcel journey while covering the shortest distance, and why every last-mile idea is really an attempt to raise drops per hour.

Each arrow is a custody transfer, and every one of them is a place where the parcel can stop moving.

The loop from the failed attempt back to the destination depot is the whole economics of the last mile in one edge. A first-time delivery costs one drop; a second attempt costs another full drop for the same revenue, so a network with a ninety per cent first-time rate has ten per cent of its most expensive resource producing nothing.

The sort steps are where the network's shape shows. At the origin depot a parcel is sorted by region because the trunk vehicles are regional; at the destination depot it is sorted by round because the vans are geographic. A parcel that arrives after the round has been built and loaded does not get delivered today regardless of how early it was collected, which is why cut-off times are a hard product constraint rather than an operational preference.

The custody transfers are also where liability moves, and each one needs a scan for the same reason a bank needs a ledger entry: without it, a loss cannot be attributed. The practical rule is that anywhere goods change hands, vehicle or building, there is an event, and the absence of an expected event is itself the most useful signal you have.

When there are many origins and many destinations and no individual pair has enough volume to fill a vehicle. Routing everything through a hub means each spoke needs one vehicle to the hub rather than one to every other spoke, so the number of links falls from roughly n squared to n and every vehicle runs full. The cost is distance and time — a parcel travelling between neighbouring towns may go two hundred miles via the hub — plus a single point of congestion where a sort failure delays the entire network overnight. Point to point wins on dense lanes with reliable volume, which is why mature networks are hybrids that bypass the hub for their strongest pairs.

A carrier owns the vehicles and moves goods. A freight forwarder owns no vehicles: it buys capacity, consolidates consignments and handles documentation, so its product is arrangement and paperwork rather than transport. A third-party logistics provider operates a function on a client's behalf — warehousing, fulfilment, transport management, or all three — and typically holds the client's stock under its own roof. The engineering consequence is whose system is the system of record. When a 3PL runs the warehouse, its WMS holds bin-level truth and the client's ERP holds an aggregate that arrives on a file, so the client's stock figure is always a little behind reality and reconciliation is a permanent scheduled job rather than an exception.