Multi-Unit Background documents --- Found this while looking at background Documents for Vogtle 3/4 --- particularly AOP-101 -> Operator Actions -> Step 2: Note 2.
When there is a unit designator (like <U-NAME>-AOP-101. ) in the note's text, PROMS renders the text and pulls it over as text only to the background document.
This causes printing the background document to print as if both units instead of resolving if a specific unit is being printed.