The WM_CLASS Property is used on X11 to assign rules to certain windows, e.g. “this is a GIMP window, it should appear on workspace number 16.” It consists of two fields, name and class.

Wayland (or rather, the XDG shell protocol – core Wayland knows nothing about this) only has a single field called app_id.

When you run X11 programs under Wayland, you use XWayland, which is baked into most compositors. Then you have to deal with all three fields.

Some compositors map name to app_id, others map class to app_id, and even others directly expose the original name and class.

Apparently, there is no consensus.