The option is enabled by default, so on startup the first task bar that is created will have this (hence the enum crap instead of a simple bool).
I think KWin at least occupies Meta+0 for it, so that cannot be assigned by default. (Forgot whether the others wee pre-occupied).
When checking the option in an applet while another one already has it, it will be unchecked for the other applet. If you uncheck it, the functionality will be disabled.
Config dialog
{F277736}
When there are multiple applets present (wording/visuals could be improved)
{F277737}
Global shortcuts config
{F277738}