Goods can be converted from a unit of measure to the items base unit of measure using the UoM chart functionality.

The base unit of measure is the unit of measure used in the accounting system and can be seen in the item details screen in The Order Exchange

For example if a retailer orders 1 carton (CT) @ $100 dollars, but your accounting system stores these as eaches (EA) at $10 each (with 10 in a carton) then you can use the UoM mapping to convert cartons to eaches by specifying the UoM:Qty:direction such as CT:10:*. 

To go the other way where you use cartons but the retailer orders in eaches (EA) you could use EA:10:/ to convert 100 units into 10 boxes of 10

This functionality will convert the quantity and the price, so it is recommended that price checking be used with this feature.