\DeclareTextComposite ¶Synopsis:
\DeclareTextComposite{\cmd}{encoding}{simple_object}{slot}
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.
\DeclareTextComposite{\^}{T1}{o}{244}
See 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
the \DeclareTextAccent{\^}{T1}{2} command.