Variable: completion-pcm-complete-word-inserts-delimiters

Treat the SPC or - inserted by `minibuffer-complete-word' as delimiters.
Those chars are treated as delimiters iff this variable is non-nil.
I.e. if non-nil, M-x SPC will just insert a "-" in the minibuffer, whereas
if nil, it will list all possible commands in *Completions* because none of
the commands start with a "-" or a SPC.