You link incoming invoices from Invoice → Incoming invoice. Show the unlinked invoices, click Link on an invoice and choose where it should go. As soon as you have chosen, the next invoice appears automatically, so you can work your way through the whole list.
How to do it
Go to Invoice and the Incoming invoice tab.
Click the blue circle above the table to show the Unlinked invoices.
Click Link on the invoice.
Choose where the invoice should go: Prod. code only, a specific Purchase, a Change Order Sub, a Counter Claim or an expense item in Setup and Operation. You see the total and remaining amount for each option.
The next unlinked invoice appears automatically. Continue until the list is empty.
Tips
Use the Comments column, which shows the text from the accounting system, to understand what the invoice concerns.
Opening the invoice is not enough. You must click Link and choose an option.
If you have many invoices going to the same place, you can link several at once.
Linking is an ongoing task. Much of it can be automated, for example with automatic invoice import on expense items in Setup and Operation.