CATS allows you to select a location screen as inventory on an invoice, effectively copying the information from that screen to the invoice and marking that screen as paid or billed out. Depending on your setting, several factors may stop you from being able to select a particular screen as inventory.

If the screen already has an invoice on it, posted or unposted, then you will not be able to select the location screen on another invoice. You can see the invoice on the screen as shown here:

Also, when in the Select Inventory scan, you can see that this inventory has already been reserved and what invoice number has been assigned to it.