Most things go over two rails.
Anything with "legs" goes on the outside rails with one leg either side of the holding hook thing, the waistband on the inside, feed it through low enough that the waistband sits just above the heated rail beneath it. So with my three tier airer I can get four pairs of trousers on it, if I have space I put them over two bars then feed through.
Shirts and blouses are put on hangers and hung on the outside sides.
Lightweight items like gym t-shirts/shorts/underwear get put on the lowest tier.
Socks hung over outside rail again lowest tier.
Things on the other tiers tend to rest on the rails below but I do try to keep an air flow between items if possible.
When I run out of space on the rails I lay larger items like hoodies at the very top (on the metal strut things) can get two on with the hoods hanging down.
The whole thing is then covered with two kingsize duvet covers sometimes pegged to hold in place.
Mine was used upstairs next to a small single radiator, in the winter found it handy when the heating was on to drape the duvets to include the rad just to help things along.
Something do need to be shuffled to dry thoroughly but the heated airer takes a much larger load than my normal airers.