Triathlon - Oval concepts aerobars

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?

I had an over under system but sold the bike I was using the faceplate with. Hell, if you are interested in an over/under 26.0 faceplate and a set of armrests, give me a holler.

But to answer you questions:
1. yup face clamp size
2. I've ridden without pads and its okay for 10 minutes at a time, but I went with the armrests myself
3. I liked the system while I had it. It's fairly light, the armrests sit low, and it's fairly easy to install

thanks triguy, I think I finally get it. Did you like it over traditional style bars where it clamps directly to the handlebars? There's a bit of a price premium and was wondering if it's worth it.

Unfortunately I need a 31.8 faceplate