Swapping your Smashpot to another fork? You'll need the right top cap for that fork. If you're changing between fork brands, you'll also need the appropriate foot stud.
Changing travel? Grab yourself an extra length of heat shrink to reapply to the cartridge tube.
Got a 2021 Fox 36 that you want to move your Smashpot over to? You'll need the 20mm top cap adapter to correct the fork travel. 2015-20 Fox 36s don't need the adaptor.
All Fox 36/Marzocchi Z1 top caps now ship with the 20mm adapter.