For example, suppose we want to transform NP(DET, N) (but not, say, NP(ADJ, N)) into the tree N using rules in T. Although this is a simple xT rule, the closest we can get with T would be q NP(x0, x1) → q.N x1, but we cannot check both subtrees without emitting two independent subtrees in the output (which rules out producing just N).
xT is a bit more powerful than T. Derivation trees for a transducer M = (Σ, ∆, Q, Q i , R) are T R (trees labeled by rules) isomorphic to complete leftmost M-derivations.