When set on an object, when a command match is done, it checks the object for attributes of the form '$<pattern>:<commandlist>'. If the command matches the pattern, then <commandlist> is executed. In other words, it checks the object for arbitrary user-defined $-commands.
TinyMUSH is often configured to require the COMMANDS flag on objects before they are searched for $-commands. PennMUSH and TinyMUX use the NO_COMMAND flag which indicates the object should NOT be searched.