Keep the supplier's product code as a reference and assign your own SKU to each sellable item or variant. A supplier code may cover several sizes or change over time, so do not use it as the only identifier for your stock.
Record both codes on the purchase and receiving notes, together with the model, variant, and batch where relevant. Check the package label against the supplier's description when goods arrive. If two items have different specifications, give them separate internal codes even when the supplier uses one family code.
Maintain a lookup between your SKU and supplier references so staff can place a repeat order without confusing similar goods. Keep old supplier codes in the history when a catalogue changes; do not overwrite the mapping if the product itself has also changed.
