The 33 includes clothing, accessories, jewellery, outerwear and shoes. It excludes underwear, sleepwear, in-home loungewear and workout clothing.
The point of the challenge is not the number but the constraint — three months is long enough to discover which pieces you genuinely reach for and which you kept out of habit.
Project 333 is one of the two sources for the widely quoted "37 piece capsule wardrobe", the other being Caroline Rector's blog Un-Fancy. Neither author framed their number as a rule.