You can assign a variable to a feature/datum entity (see "Assign/deassign a variable to/from a feature") using the same procedures as those used to assign a variable to a drive dimension or using the proper command from the Model Structure shortcut menu. If the value assigned to the variable is >0, the item is active. If the value is 0, however, the item is deactivated.
Similarly, you can assign a variable to an X-Reference component (see "Assign a variable to an X-reference component") . If the value of the variable is different from 0, the component is loaded. If the value is 0, the component is unloaded.