While people often use "adjoining" and "connecting" interchangeably, there is a technical difference in the hospitality industry:
: Rooms that feature an internal door, allowing guests to move between them without entering the hallway. Adjoining room meaning - BCD Travel adjoining room
: Rooms located side-by-side or very close to each other (across a hall) that share a wall but do not necessarily have an interior door between them. Guests must use the public hallway to move between rooms. adjoining room
An refers to two hotel guest rooms located next to each other that share a common wall. adjoining room