Hi
For a detailed answer it's best you download the Suunto User manual. However I can give you a short answer.
I use the graphical displays as opposed the the conventional (just numbers) so my answer is based on that.
Fully customize able is a bit of a lie. you cna customize the display of certain data in certain places
1. You can decide on what screens (the screens show) the primary data as a graphic and the rest as numeric) you want and in what order. For me I have NDL, GAS, Compass, and Timer.
2. In each screen additional data is displayed around the edges. So for instance on all screens except the gas screen I have my gas contents. The bottom right position you can have a number of values that you can scroll through, such as max depth, dive time, gas time, gas type, consumption, PO2, CNS, OTU and the like
So in each screen I have some of the same info and some different info depending. For instance My NDL screen will be the one I end the dive on, so as well as showing gas pressure in one area and in the bottom right it will have stuff like dive time, Max depth etc etc - the data I'll need when I give my stats when boarding the boat
My Gas screen will have stuff like OTU , CNS, PO2
You set it up with the data you want to see where. Its easy at first to just tick every box - but then you find it's a PITA to scroll through everything. You will soon find suits you in what data is displayed where.
Hope that helps as a starter