I've been looking to put clip on aerobars on my bike and was looking at the oval concepts stem cap system.

luckily the bike came with an oval concepts stem when I bought it so naturally I decided oval concepts but I need some clarifications on all their options.

What do all these over/under, over/over, under/under terminologies mean? I assume it's the position of the aerobars and elbow pads with respect to the handlebars.

Then I see it comes in 26mm or 31mm, meaning that the stem cap is for 26 or 31mm handlebars?
I saw on slowtwitch that some people don't bother with the elbow pads, wouldn't that be a pain on the arms?

What are some experiences and setups that you've chosen with this system?