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Microsoft Windows O/S Licensing
Each PC running Microsoft Windows requires a license for the operating system. New PCs from major vendors typically include a Windows license for that PC, but some “bare bones”
Desktop Editions
Windows 10 or Windows 11 Home, Professional, and Enterprise are all “desktop” licenses for the O/S.
A PC running a desktop edition of Windows is licensed for access by just one user at a time. The user may connect either locally at the screen and keyboard, or remotely using Windows Remote Desktop Protocol (RDP).
If a user tries to connect to a desktop Windows O/S where a user is already logged in, the user will see a warning that to connect they would first had to end the other session. (If the same user is re-connecting via RDP while already logged in locally, or vice-versa, the connection goes ahead and the existing session is shifted to the user's new connection.)
On a desktop edition of Windows, it is possible to create user logins for a number of separate users, each with their own login name and password. A different user may log in and use Windows on this PC, but only after any other user's session is closed. Multiple users need to “take turns” accessing the PC one at a time. This limitation covers both local and remote logins. One user cannot be logged in remotely while another is logged in locally.
Server Editions with Concurrent Session Support
If you want to operate a computer under Microsoft Windows and allow more than one user to connect at once, you must buy a license for a server edition of Windows for that PC. There are additional license items required for “Client Access Licenses” (CALs) on a per-user basis, depending on what services are offered by the server for users to access.
Contrast with Unix and Linux based O/Ses
The restrictions on one user logging in at once do not apply in other O/Ses such as Linux, Solaris, or Mac OS. Each of these supports one or more remote login sessions at the same time in addition to one local login session on the screen and keyboard. This option can be configured by the system administrator to control which users may connect, or to disable remote access, but there is no technical nor legal issue to prevent concurrent access in these O/Ses.