Of the 4 months I was on reserve, I was given a RAP or standby assignment out of seniority order at least once in each of those months (perhaps more than that but those are the times I noticed). I think there is supposed to be some operational necessity (like it makes no sense to give you RAP3 on your last day would be an obvious one but there's other scenarios less obvious). The first time, I could not even understand the reason so I called to complain to no avail. The third time, they gave me a Standby 1 when I proffered for RAP2 and the junior guy that got RAP2 had the same schedule as me. The second and forth time, I could guess an operational necessity by looking at the reserve list.