Confusing - yes it is! 🙂

OK, I won't spend any more time on the T-SQL side. But I haven't quite finished.

Your FieldB in TableA appears to be an FK to either of FieldA and FieldC in TableB - that's pretty bad design & a little more difficult to enforce than it needs to be

Why not create a new column on TableA which is a pure FK to TableB.FieldC?