\ProvideTextCommandDefault , Up:
fontenc package [Contents][Index]
Access an accented glyph directly, that is, without having to put an accent over a separate character.
This line from t1enc.def means that
\^o will cause
LaTeX to typeset lowercase ‘o’ by taking the character
directly from slot 224 in the font.
fontenc package, for a list of common encodings. The
simple_object should be a single character or a single command.
The slot argument is usually a positive integer represented in
decimal (although octal or hexadecimal are possible). Normally
\cmd has already been declared for this encoding, either with
\DeclareTextAccent or with a one-argument
\DeclareTextCommand. In t1enc.def, the above line follows