At it's heart, an Alias is just another SKU for the same item. If you have a product with 3 SKU's you could choose one of them and say it's the "real" SKU or "Primary" SKU and the other two are Aliases for it. In GroovePacker all SKU's for a product are basically interchangeable. The only thing that sets the primary SKU apart is that it's the one displayed in lists and in Scan and Pack.

We've given each product in GroovePacker a product ID so that we could keep track of it while SKUs can be added and changed. Each ID number represents a physical item and is displayed at the top of the product window when you are viewing the product details. Normally the ID isn't thought of or mentioned. In fact it only really comes up when we talk about aliases.

We can associate as many skus (and barcodes) with an ID as we like. 

The only restriction is that each SKU and Barcode assigned must not already be assigned to another product ID.

This makes it impossible to have two different product ID's with the same barcode. This is a good thing, because if two items that were actually different shared a barcode it would be impossible to know which was scanned and shipped and which to deduct inventory for etc.

If you have two SKUs that are really aliases for the same product then they will need to be scannable using the same barcode. In this case the barcode and both SKUs are added to the same product ID. Now when either of those SKUs appear in an order you can scan them using the same barcode.

The only case where this approach will not work is when you actually have two physical items, requiring separate IDs, and separate inventory counts. In that case you can not share a barcode between them. 

If you're creating or editing a CSV to import your products you'll want to setup your file with columns for each SKU. The column headers will appear as follows:

SKU1 | SKU2 | Barcode | Other data | Other data

More info on importing Aliases via CSV and be found here.