FBL5N displays customer line items. An open customer item is a receivable that has not been cleared at the key date. Billing creates the receivable; collecting a matching payment clears it. Those events have different effects on revenue, so a collection must not be treated as a second sale.
Filter by customer and status to answer a specific collection question. The same report can also show a paid item when you select cleared status. Dates matter: a payment after a reporting date should not erase the fact that the invoice was open at the earlier date. In these exercises, work from the prepared billing records and use the displayed amounts. The hidden data changes opening invoice amounts, which makes a memorized total unreliable.