Simply put, it means that a bus that is not assigned to serve on
a particular route is doing so. "Perm" is somewhat the opposite: it
means a bus has been assigned to serve a particular bus route. Some
buses may be a perm on more than one route, sometimes with special
arrangements (for example, it will serve this route on weekdays,
but on weekends, it will be assigned to another route).
All spare buses (buses not assigned to any particular route) are
cameos when they are used for revenue/passenger service.