4.1.4 \DeclareTextCommand & \ProvideTextCommand

Synopsis, one of:

\DeclareTextCommand{\cmd}{encoding}{defn}
\DeclareTextCommand{\cmd}{encoding}[nargs]{defn}
\DeclareTextCommand{\cmd}{encoding}[nargs][optargdefault]{defn}

or one of:

\ProvideTextCommand{\cmd}{encoding}{defn}
\ProvideTextCommand{\cmd}{encoding}[nargs]{defn}
\ProvideTextCommand{\cmd}{encoding}[nargs][optargdefault]{defn}

Define the command \cmd, which will be specific to one encoding. The command name cmd must be preceded by a backslash, \. These commands can only appear in the preamble. Redefining \cmd does not cause an error. The defined command will be robust even if the code in defn is fragile (see \protect).

For example, the file t1enc.def contains this line.

\DeclareTextCommand{\textperthousand}{T1}{\%\char 24 }

With that, you can express parts per thousand.

\usepackage[T1]{fontenc}  % in preamble
  ...
Legal limit is \( 0.8 \)\textperthousand.

If you change the font encoding to OT1 then you get an error like ‘LaTeX Error: Command \textperthousand unavailable in encoding OT1’.

The \ProvideTextCommand variant does the same, except that it does nothing if \cmd is already defined. The \DeclareTextSymbol command is faster than this one for simple slot-to-glyph association (see \DeclareTextSymbol)

The optional nargs and optargdefault arguments play the same role here as in \newcommand (see \newcommand & \renewcommand). Briefly, nargs is an integer from 0 to 9 specifying the number of arguments that the defined command \cmd takes. This number includes any optional argument. Omitting this argument is the same as specifying 0, meaning that \cmd will have no arguments. And, if optargdefault is present then the first argument of \cmd is optional, with default value optargdefault (which may be the empty string). If optargdefault is not present then \cmd does not take an optional argument.