Ill be applying for it in about 2 weeks.
I think they said a photo, a letter explaining why you want the extention, (travel for example) copy of passport, give them the old ARC, and I think there was something else which I forget.
A week or so later they give you a new arc valid for 6 months, with no purpose written on it.
The nice thing is that it does not interrupt your PARC eligibility