^ Both. Sometimes the controllers knew we can do it and assign it, sometimes they ask us if we are qualified and sometimes we have to request it.
Normally if the weather has been down for a while, everyone knows we are capable or we would have diverted. Then it usually just gets assigned.
Plus sometimes we request in better than Cat III weather because we have to do them for currency.
It's been a while since I've done any because I'm flying interisland now. I go to 767 training next month and will be doing them again by winter. Oh boy.....