Basically, they will assign trips from bucket 1 first, for trips that match days of availability. Assuming a 3-day trip(s) in open time, if they have more trips than pilots (or if they don't have any bucket 1 pilots with 3 days of availability), they will go to bucket 2, then bucket 3, then bucket 4. THEN if they still haven't found a pilot, they will start using the 4-day pilots from bucket 1, then...

The Scheduling Reference Handbook, page 64 has a pretty good description.

RAW score within a bucket means nothing.