Similar to control group, but you don't need to select it, as it is already selected. The initial idea is that your primary selection is white, while you secondary selection is black. Black will not work as it would make the unit icons unreadable. I'm still debating it with myself and probably will not have this resolved until I finish all global commands, which would remove the requirement to select factories(is there a widget that can control factories without selecting them), workers(global build command does most of the work, just need to not require a unit to be selected, and building priority could use some work.), missiles(done), supers(lowest priority of the global commands, but also probably the simplest)
Maybe some things can't easily be done with a global command, such as "hold fire so nano towers can repair you faster then the artillery can damage you"