In e-commerce, it is a common situation to find one product listed under a different SKU depending on the marketplace. For example, you might have a widget listed on Amazon with the auto-assigned SKU they provide, while it may sell on your own website as your own internal SKU. In your inventory system, ideally each of these identical listings will translate down to a single inventory item, so you can efficiently fulfill orders without the confusion of having to cross-reference SKUs during the fulfillment process.
How this is handled in our system is a creative use of the "kit" function. You simply create a 1-item kit to point any secondary SKUs ("aliases") to a single internal SKU that you use for inventory purposes. An example: Let's say you have a product for sale listed on Amazon as "SKU2" and on your own website as "SKU1". You can set "SKU2" up as a kit, containing a single item in its Bill of Material: Item "SKU1" (qty: 1). When orders arrive for SKU2, they'll get automatically translated/fulfilled/updated for inventory purposes using item SKU1.