Thanks, Adam. That link is much better, and I'm happy to follow your advice on wording as you're our number one criterion lawyer ;-)

Here's an updated criterion proposal. I marked adjusted parts with an asterisk.

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User switching

User switching must work using the mechanisms offered (if any) by all release-blocking desktops in their default configuration.

What is user switching?
User switching is a process of changing the currently presented desktop session between concurrent sessions of two or more different users. The user sessions keep running in the background, and users can switch between them repeatedly without losing any running application state.
For the purpose of this criterion, user switching doesn't include switching between different sessions of the *same* user.

Work?
The switching mechanism must correctly perform* the requested operation. If the operation doesn't succeed* on a subset of graphical drivers, the release blocking decision should be based on the number of affected users, the problem severity and available workarounds (as is our [standard procedure](Blocker_Bug_FAQ#What about hardware and local configuration dependent issues?)*).

Default configuration?*
This is supposed to cover only cases where the system hasn't been modified in a substantial (and relevant) way. This excludes cases where people e.g. install multiple desktop environments, replace their display manager for a different one, tweak relevant systemd settings, or install a non-default graphics driver.
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* denotes a change in wording