1. Go to the song you would like to edit, tap/click on the Edit icon.
2. Tap/click on the first measure where you'd like the rehearsal mark.  
iPhone/iPad: Tap the Rehearsal letter button along the bottom row of buttons (4th button from the left) then choose from the popup menu. (New Editor: tap on the Rehearsal letter to remove it)
Android: Tap on the rehearsal mark 'A' several times to go though the available options.
Mac: Click on the appropriate Rehearsal mark.
The IN (intro) and V (verse) rehearsal marks indicate a section in the song that will only be played once at the very beginning.

Subsequent choruses will start after the intro or verse indicated by the presence of an A, B, C or D rehearsal marks. However use of the D.C. (da Capo) will return to the beginning of the song as expected.

In addition, many of the styles will introduce variations when going from the A section into the B,C or D section (most of the Pop and some of the Latin and Jazz styles support this). The dual styles (like Latin/Swing or Swing/Two Four) change between the A and B sections for the first and last chorus.

Most of the time, a drum fill will accompany the transition from one section to another.