Created attachment 128348 [details]
Screenshot of Display Configuration module showing no screen numbers, in context with a Window Rules setting where the screen number is needed

SUMMARY
For each connected monitor, the display configuration module helpfully provides the make and model and interface-related identifiers (eg HDMI2, VGA-1-0, HDMI-1-0).

Those pieces of information are shown both in the draggable preview of the screen screen layout and in the on-screen identifiers which appear on each screen when you click the Identify button.

However, the module does not show what screen number is associated with each monitor.

STEPS TO REPRODUCE
1. Look at details shown in screen layout preview.
2. Look at on-screen identifiers which appear when you press the Identify button.

OBSERVED RESULT
No screen numbers are associated with the monitors in either case.

EXPECTED RESULT
I would expect the screen number of each monitor to be shown.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.2
Kernel Version: 5.4.39-1-lts
OS Type: 64-bit

EXAMPLE OF WHY KNOWING THE SCREEN NUMBER IS USEFUL
In the System Settings Window Rules module, under the Size & Position tab, it's possible to set "Screen" (which might be more helpfully called "Screen Number") as a property which can be forced or applied to a matched window. But to set this, one needs to know which screen number applies to which physical monitor. The Display Configuration Module is the obvious place to look for that information.