The first dropdown has two options: front and back. Due to this, this block can make sprites go to the very front or back of the whole stage. This makes it useful in a wide variety of situations.
Preparing for a race:
when I receive [setup v] go to x: (-50) y: (-100) go to [front v] layer switch costume to [car v] . . .
Starting a platformer:
when I receive [play v] go to x: (-220) y: (-60) go to [front v] layer start platformer script::custom . . .
In some sprites, a sprite needs to move forward or backward, but not to the very front or back. The
go [ v] () layers block does that, with an input to specify how many layers the sprite goes forward or backward.