A Supplier invoice is a Sales invoice from a Supplier. It can be used to either record details of an invoice issued by your Supplier in respect of a Receive, or it can be used to record details of an invoice issued by another Supplier for non-stock items.
A Supplier invoice can only be created for one or more Receives for the same Purchase Order. If a supplier has issued a single invoice that covers multiple Purchase Orders, a separate Receive and Invoice transaction must be created for each Purchase Order.
A Supplier invoice is the actual price that is being charged by the Supplier for goods, and overrides (and recalculates) the prices for any Receives that were created using the original Purchase Order cost.