It's for both PvP & PvE.
Why is it best to go Deception first?
I would not advice starting your gear grind as deception unless you run with a premade or you have a lot of patience. Deception is heavily gear dependant and you will be quite squishy until you are in at least full BM. Your burst(which is really one of the only benefits of deception builds) will be pretty lack luster as well. If you are gearing through solo queues then you may want to spec either full darkness, or a darkness hybrid until you've cleared most of your recruit pieces. Once you hit a good mix of BM/WH gear, you start to see the benefits of a full deception spec. If dying very quickly and kill sniping being your only true means of downing your opponents doesn't bother you, then roll deception from the start though.