A bill plan is the rules for producing an invoice. When, for how much, in what format. A revenue plan is the rules for recognizing revenue. When, and how much.
THEY ARE SEPARATE AND THEY SHOULD BE. You can invoice in advance and recognize revenue as work is done. You can do work and recognize revenue before you are entitled to invoice. The two events answer different questions: WHAT CAN WE ASK THE CLIENT TO PAY, and WHAT HAVE WE EARNED?
And the two balances that separation creates. Unbilled — revenue recognized, not yet invoiced. An asset. Unearned — invoiced, revenue not yet recognized. A liability. THEY ARE ON THE BALANCE SHEET, THEY ARE AUDITED, AND EXPLAINING THEIR MOVEMENT IS A MONTHLY TASK. That is why the project accountant lives with these two numbers. They are not a reporting curiosity; they are a balance sheet position somebody signs.
The design consequence: you configure a bill plan and a revenue plan separately, and they can use different methods. That is not a mistake to be corrected. It is the point.
MANY CLIENTS ASSUME INVOICING AND REVENUE ARE THE SAME EVENT. Correcting that assumption early prevents a difficult UAT — because the alternative is Finance looking at two numbers that differ and reporting it as a defect, which is failure case four.