How to mount the 40T expander:
There are two ways to fit 40T sprocket to your drivetrain. You can remove the stock 17T sprocket and one spacer from your cassette, then put 40T on the freehub and mount everything else back (except 17T).
OR, by removing 11T sprocket. Second option is a lot better in our opinion, but requires our special lockring to hold 13T sprocket (lockring is available in options).
If you don't know which option would be better for you, then just go for a ride and observe which of these two sprockets (11 & 17) you will miss less. Study shows, around 90% of mtb riders do not use 11T sprocket in terrain.
*SLX - this cassette only allows to take 11T and use our lockring to lock on 13T sprocket.