\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 location 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.