MVA base used in transformer parameters

[Niccolò Citroni]

A simple question which is not specified in the caseformat file: for branch
parameters (epecially in regard of transformers) I have to define x r and b
in p.u. Should i use p.u. values referred to the MVAbase of the case or
p.u. values referred to the specific MVA base of the transformer (or line),
and then matpower automatically omogenizes them?
Thakyou very much,
Niccolò Citroni