TL;DR: I hope this is universal, worth a shot.
I used this code for a purchase recently, whilst signed in to my Samsung account. I then used it again whilst checking out as a guest. I then trialled it once more as a guest (after both purchases went through) to see if it could be used again and the 20% was still applied once more.
It seems this can be used across the entire store, and is not device/accessory specific.
For perspective, combined with the trade-in deal (broken S8 - £205), I purchased a Galaxy S21 Ultra 256GB Phantom Titanium and Galaxy Buds Pro (there's also another discount here for buying both at once) and 2 years of Samsung Care+. My basket total was around £1040 with total savings exceeding £500. I have a feeling you could save another £100+ if combined with the recently posted cashback deal.