For me, the "casual" driver is nothing more than a temp. Fresh meat to be exploited in management eyes. Although, in all fairness, I have seen some casuals during peak that refused to be exploited, knowing full well that they would be gone soon. They were given easy stuff to do and not too many stops.
The "cover" driver, aka, "split" driver in my parts, is a regular-union driver, without a bid route. They fill in as needed to cover bid routes and splits of the day.
What gets confusing is when a part timer becomes a casual during the freeze period. They are still union members but not working towards seniority. This is only temporary, so I consider they a split driver since after the freeze, they will be working towards seniority.