When a product is set up with multiple UOMs for purchasing and sales, particular attention is needed for inventory movement (purchases, sales, adjustments) to keep your inventory accurate.
When adding a UOM in addition to the default UOM, the whole unit will determine how your product stock is managed. If Whole Unit is set to true, then only whole units of the larger UOM will be stocked and the smaller UOM will consume partial quantities. For example; a product with a default UOM of BOX can also be sold in pieces (PCS). There are 50 pieces in a box.
With the whole unit set to true, if a box were broken open and 10 PCS were sold then there would be 40 pcs in stock. If whole unit was set to false, then there would be 0.8 BOX in stock.
Product Return
When selling a box and the customer returns a portion of the box, if return policy allowed this, then a product return cannot be against the sale invoice due to UOM change. A POS return would need to be performed. A return can be performed on a POS order by changing the UOM on the return and updating quantity.
Inventory Count
When performing an inventory account pay attention to the UOMs when counting as if you have both BOX and PCS then both will appear on the count. The first two rows below are for whole unit = true and the last two are for whole unit = false. If you manage to put a BOX back together with the PCS and counted, in this case 50 PCS, then when you counted the system adjustment would adjust in a BOX and adjust out 50 PCS (give or take depending on how many PCS were counted) as they were not counted as PCS.
Purchasing
On the purchasing side, when creating the purchase order the UOM will be pulled from the product unless you are procuring it from a work order, then it will match the work order UOM. Make sure the UOM, quantity, and cost are in align to sure inventory accuracy.