\DeclareTextAccent

Synopsis:

\DeclareTextAccent{cmd}{encoding}{slot}

Define an accent, to be put on top of other glyphs, in the encoding encoding at the location slot.

This line from t1enc.def declares that to make a circumflex accent as in \^A, the system will put the accent in slot 2 over the ‘A’ character, which is represented in ASCII as 65. (This holds unless there is a relevant DeclareTextComposite or \DeclareTextCompositeCommand declaration; see \DeclareTextComposite.)

\DeclareTextAccent{\^}{T1}{2}

If cmd has already been defined then \DeclareTextAccent does not give an error but it does log the redefinition in the transcript file.