Like many screenwriting programs, in Fountain Mode pressing TAB will do the most convenient thing based on context.
The most common use is triggering autocompletion. If the point is at a
blank line or the end of a non-blank line, pressing TAB will
completion-at-point. See Autocompletion.
In Fountain Mode, TAB is also used to control outline visibility. So
if point is at a scene or section heading, it will cycle visibility of
that scene of section between collapsed and expanded. To allow for
more control over outline cycling, if TAB is prefixed with
fountain-outline-cycle and pass
TAB also helps working with parentheticals. If the point is at a blank line within dialogue, it will insert a parenthetical; if the point is inside an empty parenthetical, it will delete it, or if inside a non-empty parenthetical, move to a newline.
If the point is at or inside a note, TAB will cycle visibility of that note between collapsed and expanded.
This all might seem complicated, but the idea is by covering all the cases you don’t have to think about it.
This is the command you’ll use the most. Bound to TAB, it will perform the most convenient action based on the current context.