Looking at the rules for draft legal triathlons, this is from World Triathlon Rules:
For the Elite/U23 do you think the maximum rim dimension of 25 mm, refers to the depth?
Looks like for AG, the rule is less strict. I’ve got 50 mm depth wheels so I’m hoping that’s legal.
Nah, 25mm on each side, so given it’s a circle, which only has one side, which means it must be talking about left/right, so it’s saying the maximum tyre width is 50mm…
What awful rules! Steel wheels allowed complete with their worse braking than carbon!
So actually I guess, they’re trying to take the UCI rules, but have rewritten it to be confusing -
“A traditional wheel is defined with a rim height of less than 25mm, a rim in aluminium, a minimum number of 20 steel spokes which are detachable, and all components must be identifiable and commercially available.”
Which is quite a bit clearer - aluminium for the rim, 25mm explicitly talking about the height, spokes have to be steel.
Thanks 
the Elite rules are talking about a 25mm rim depth - i.e no deep rim wheels. This is to ensure race wheels conform to a standard so there is a level playing field for everyone competing. We use the UCI list to check all bikes and if not listed the Head Referee (or Chief TO depending on event level) will make a decision to allow or not. The UCI wheel list ensures that all listed wheels have been crash tested and are safe to race on - some cheap ones are frankly abysmal in a crash.
For Age Groupers, the rules are a little more relaxed so you can have deep rim wheels provided they comply with the 12 spokes rule and no disks or tri/quad spoke wheels.