We don’t use CBS but use this field an additional flag rather than giving ‘Priority’.  EG The Red and Green drive conditional formatting on the Work Instruction so the technicians can easily spot the differences between these and standard production jobs.  We use them in another area to ‘Group’ jobs by particular sets of Work Centre.  

I think it’s one of those you can use and almost make up your own rules as to what they mean/action.  

Don’t know if that helps?