ATT,I'm a Chinese beginner of electric guitar,and I don't know the meaning of  ‘9V sag unit’ on the pedal power supply.

Anyone can tell me? Thanks. 
In the context that it's used, if used properly would mean that the voltage drops as the current draw rises.  Basically it means a reduction under load.
If your boobs are tucked into your pants then your SAG is probably set to low.   

Quote by Desert_xuan
JELIFISH19 By the way, would you like to explain the meaning of the word "sag" to me ? Thank you again.

SAG = Screen Actor's Guild.
See, AFTRA time, The battery will lose voltage and your sound will "bulge downward under weight or pressure or through lack of strength" as do the body parts on some screen actors. Their voices change, and that's where pedal makers got the inspiration for it. If you've got a toob amp, it refers to the drooping of the power supply voltage in response to large transient signals, which lends a certain dynamic "feel", and as we all know, screen actors are all about the feelz.