here are different frames, which one represent +, -, etc
It doesn't matter. "master_volume" is a global variable so "System: Set "" volume to master_volume db" refers to global variable.
Every tick -> System: Set "" volume to master_volume db
means
if you set master_volume do 1 then volume will always be 1
if you set master_volume do 5 then volume will always be 5
You only need to specify when to add or subtract values from that variable.
It's like for example opacity for a sprite
gv "spr_op = 50"
button up -> add 20 to spr_op
-> set sprite.opacity to spr_op
button down -> subtract 20 from spr_op
-> set sprite.opacity to spr_op
this is equal to
gv "spr_op = 50"
every tick -> set sprite.opacity to spr_op
button up -> add 20 to spr_op
button down -> subtract 20 from spr_op