Relaxed attire can mean anything like wearing a vest or sweater in lieu of a jacket, wearing 1/2 chaps.
If you’re doing a 2’ class I’d forgo boots completely. I also would probably forego a 2’ jumper class unless you plan to ride it like a hunter trip.
one of my pet peeves = green horses in divisions meant for novice riders. Please read the division or class specs for each height. I don’t like to see lower divisions used as baby green hunter divisions. If it’s a 2’ schooling hunter class fine, but if it’s 2’ pleasure hunter or novice/long stirrup hunter - please don’t take your green bean in the U/S class (OF is fine) . Sometimes you get these green horses using a pleasure divisions as a “baby green” hunter and you’ve got novice or inexperienced riders and a green horse that may blow up sometimes its not a pretty sight.
You’ve gotten some great advice here. Enjoy hope you and your horse have a great experience.