the only one that matters is the one after the word "cast" - if you're doing a long macro and pushing the 255-character limit, the spaces between brackets and spell names can be removed, like so
Code:
/cast [@mouseover,help][]Heal
FYI, the "help" flag only attempts to cast on a friendly mouseover target that exists (yes, the "exists" flag is included automatically in both "harm" and "help").