Give a default definition for cmd, for when that command is not defined in the encoding currently in force. This default should only use encodings known to be available.
It uses only an encoding (OMS) that is always available.
\DeclareTextCommandDefault should not occur in the encoding
definition files since those files should declare only commands for use
when you select that encoding. It should instead be in a package.
As with the related non-default commands, the
\ProvideTextCommandDefault has exactly the same behavior as
\DeclareTextCommandDefault except that it does nothing if
cmd is already defined (see \DeclareTextCommand & \ProvideTextCommand). So, packages can use it to provide fallbacks
that other packages can improve upon.