How to filter records in xml using map functiods?

There is a mapping in Biztalk from schema A to schema B.

During this mapping, I would like to filter out some messages if the value in one tag is NOT in a list of values.

Any idea how to perform this filtering action in mapping? Such that some messages can be mapped from schem A to schema B (if the values inside one tag is in the list of values), but other messages will be filtered out (if the values inside one tag is NOT in the list).

Thanks a lot.