Mailing address. Several of my clients are in long term homes and they use their home or family members' address. If your client has no family/no dwelling, then probably his only address is that at the Long Term Facility. Some of my clients have been using P.O. BOX addresses and never had a problem with the CRA.
BTW, over 10 years ago one of my clients was in a LTCH and since he had no other address, all his letters were coming to the LTCH. Unfortunately, a lot of his mail went missing, presumably misplaced by the LTCH staff.