When set_modify_command() was called, the parser won't expand the standard abbreviations n,e,s,w,nw,sw,ne,se for that user anymore, nor use any hook set for this.
0 as argument will stop the command modification and reinstall the standard abbreviations. -1 as argument will just return the object previously set.
The return value is the object that was previously set with set_modify_command(), if any.
This mechanism is intended to expand aliases on quicktypers or the like.