My Filemaker User manual explains that the match field in a single criteria relationship is connected to its related table by a single thin line if only a single record can match (e.g. a serial number). The line connecting match fields has a little "fork" symbol on the end if it is possible to match multiple records.

What does it mean when the line connecting Match Fields has a little "T" at the end? I have some tables with this symbol and they don't seem to work.

Thanks!

It means the relationship only goes one way. The field with the "T" cannot be used as the child key, only the parent key. It is either a global or unstored calculation field which cannot be indexed.