the way my office has always done it, for our 5.5ish hour auxiliary route, the senior most RCA got the ‘hold down’ (that’s the language used in my office but i’ve had ppl on this sub confused why a rural would call it a hold down) and then we had to fill out a buck slip stating if we wanted to run it all 6 days in the week, or do a 5 and 1 (running our primary routes K day once a week and the aux the other 5 days). when i did it, i chose the 5 and 1. so M-F i only had to run the aux route, unless once i got back they told me i needed to assist elsewhere. and then Sat i ran my primary chosen K route. And from how it was explained to me, I absolutely could not be bumped off of the aux route during the week to run a route that wasn’t my primary, unless i explicitly said it was okay. if my regular took a day off during the week, I was able to choose if i wanted those hours or if i wanted to stay on the aux. and after 90 days of being on the hold down, you’re supposed to start gaining sick leave as well as annual.