First, Second, Third AD, Oh My where will it all end. Knowing who is who is pretty important when you are on set as an actor or background extra.
The first Assistant Director (First AD) reports to the director of the production and is there to “keep the production on schedule throughout the day, communicate to the entire crew, and to maintain the safety and security of the staff and shot itself”
Depending upon the size of the production, there may be a Second AD and /or a Third AD. The Second AD will report to the First AD and “serves as the backstage manager, liaising with actors, putting cast through make-up and wardrobe, which relieves the first of these duties”
If there is a Third AD, they are normally responsible for crowd scenes, getting extras to and from set at the correct time.
So, depending upon your role in the production (Actor or Extra) and the size of the production your main point of contact could be any one of these people. That’s the person who will let you know when you can go on set and leave set. That’s the person who will usually arrange for sign in and sign out sheets.