A + B means 'A is the father of B'
A % B means 'A is the husband of B'
A # B means 'A is the daughter of B'

If the expression is U+S%M#T, then how is S related to T?

Let's decode the coded relationship expression U+S%M#T step by step from left to right:
U + S: U is the father of S.

S % M: S is the husband of M (S[Male]⟺M[Female]).

M # T: M is the daughter of T.

We need to find how S is related to T:
Since M is the daughter of T, and S is the husband of M, S is married to T's daughter. Therefore, S is the Son-in-law of T.