Let's say that you have 3 computers but each one is a stand alone computer. Your password is checked by each computer independently. In most cases, you only need to have the computer you are using on and everything will work. On a domain, there is a server, and the other computers are workstations. When you log on to the workstation, the server verifies your user name and password. In the case of ATX, you install the server software for ATX on the server and most likely that server holds the clients' data. If the server is down, you don't have access to your clients data. The advantage is that everybody in the office uses the same data repository and everybody has access to all the clients (if needed).
You don't need a domain to run ATX server but the computers need to be connected to each other and be using the same IP subnet, but having a domain is better when you a few computers.
Technically, you can install the server portion on laptop running windows 10/11 and that becomes, NOT the server for the domain, but the server for ATX. Then you can connect other desktops, laptops or servers to that laptop and they are the subordinates of the laptop running the ATX server software. Keep in mind that the laptop still reports to the domain server (controller). Please ignore this last paragraph if needed.