I don't think there is any difference per se. I think it's just in the "language", but I haven't been around that long. My steward was a casual (20 yrs plus driver) when she started and basically did the same job that I'm doing now.
A casual driver refers to he or she not having fulltime seniority. A casual driver is a cover driver. However, any fulltime driver works before a casual driver. Ex: A center can have say 40 bid routes, 43 fulltime drivers and say 4 casual drivers. Hope this helps