\DeclareTextSymbolSynopsis:
\DeclareTextSymbol{\cmd}{encoding}{slot}
Define a symbol in the encoding encoding at the location slot. Symbols defined in this way are for use in text, not mathematics.
For example, this line from t1enc.def declares the number of the glyph to use for «, the left guillemet.
\DeclareTextSymbol{\guillemotleft}{T1}{19}
The command \DeclareTextCommand{\guillemotleft}{T1}{\char
19} has the same effect but is slower (see \DeclareTextCommand & \ProvideTextCommand).
See fontenc package, for a list of common encodings. The slot
can be specified in decimal, or octal (as in '023), or
hexadecimal (as in "13), although decimal has the advantage that
single quote or double quote could be redefined by another package.
If \cmd has already been defined then
\DeclareTextSymbol does not give an error but it does log the
redefinition in the transcript file.