Both...haha...

"within each group, rank those pilots electing to be called first, senior to junior, followed by the remain- ing pilots, junior to senior."

So if you want to fly, its senior to junior, not wanting to fly its the inverse. But as stated, If I have 3 days available and a junior pilot has 4 days available and a 3 day trip comes up, my phone will ring. If I have 3 days available and a junior pilot has 3 days available and a 3 day comes up, they will get the call, even if they just came off a trip a day prior. The one caveat, sometimes rare, is if the junior pilot will credit about 72 hours, then the schedule can bypass them for another pilot.

SlimBob basically said the same thing I did.
I dont know what pilotnicco means by "fewest days available first"