Alternates allow you to add multiple versions of any line into the script, so that you don’t have to lose formatting by adding it to a comment, or delete other versions of the line before you’ve decided on a winning version. Functions for line alternates are located under Drafts > Line Alternates, but the power is really in the shortcuts (Note: Alt and Option keys are interchangeable):
Alt-Enter: Create a line alternate for a line. If line alternates already exist, this will show/hide them.
Shift-Alt-Enter: Show/hide all of the line alternates in the script at once.
Alt-Space: Change the highlighted alternate to be the primary choice for that line (thus, switching out the actual line to an alternate).