Customize the list of related records in an input form

I have an input form for adding records to table B, that contains 2 columns: a reference to table A and a boolean. The key for table B is a computed key concatenating both columns.
In order to prevent duplicate input, the form should show a list of B records compatible with the existing A records and with the choice of the boolean.
Is there any way to customize the list of available related records in the form?


Thanks a lot, adding a Valid_If expression using list Subtraction was the solution!

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