In MOTA we have minimum 40 section attempts (usually 8 sections x 5 loops or 10 sections x 4 loops)
As far as severity goes (and this is just a generalization)
Beginner = In the start and find their way to the end, usually a turn or two (class is for learning section markers, etc..)
Novice = couple of turns and a small log or two, all can be ridden without touching the clutch
Intermediate = medium logs, tighter turns, some clutch control needed, turns on hill
Sportsman = most difficult half of Int. and easiest half of Adv. sections
Advanced = Big obstacles with plenty of run up/out and places to put feet, tight turns (hopping not necessary but makes it easier), steep hill climbs, etc..
Expert = Big obstacles hardly any run up/out with less spots to foot, tight and super tight turns, and a couple of sections that are "Screw em they are the top guys and can't get any higher in class" make it hard!
We do try to set the event for the middle of the pack for each class, while trying to maintain somewhat of a standard of difficulty in each class. Middle of pack should be able to get 2 or 3 pts on the section first loop and then progressively improve as the day goes on with cleaning most sections for the last 2 loops or so. We want the good riders to move up a class and keep the lower riders from dropping out/getting hurt. It does not really help the newly moved up riders if the sections are adjusted too much so that basically the Advanced lines are just Intermediate lines that year because a large group of riders moved up a class, they moved up for more of a challenge....... I hope that makes sense.
Of course factoring in weather conditions, available traction, etc... you want to have a challenging and fun yet safe event.
I just moved up to Expert class an event or so ago, Now my goal when riding is to finish the event with less than 200 points (200 is maximum available for those keeping track) Until I get better at riding I will have absolutely no problem with requesting a 5 if a section scares me. If that did bother me I would stay riding in Advanced.
I have been told by Biffsgasgas that his experience in an event in Europe was a great and fun time but the club level sections are set a bit harder than he has ridden in the States, so there is some "cultural differences" there.