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 call
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 one
or more of ARG, this will call fountain-outline-cycle
and pass ARGs. See Outlining.
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 it is inside a non-empty parenthetical, it will move to a newline.
If the point is at or inside a note, TAB will cycle visibility of that note between expanded and collapsed.
All this might seem a bit complicated, but in practice you can hit TAB and it will usually do what you mean, so 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.