Normally you are entered into the lowest section if you have a national rating normally they will convert that to FIDE normally that is taken at par or national - 100 pts for most countries
If you don't have a rating you don't belong to any Underclass as you are unrated and most tournaments only allow you to win the unrated prize or a Top placing prize if one large section
chess.com ratings have no value for OTB tournaments
you may also want to be reading up on FIDE tournament rules ie) touch move, handling the clock, proper draw claims, etc.
If say I were to participate in OTB tournament and want it to win prize $ , but I never played any FIDE tournaments, so I don't have an official rating.
Which "U" section do I belong to? Do I choose one or do they choose for me or how this works?