Originally Posted by scambo1

Eggzackly!

PBS is a machine and thinks like a machine. Bid what you don't want - or what you want to avoid the most- first. Then when you bid everything you do want, bid what you want the most first in descending order. If you are junior, keep your bid simple.

The machine tries to give you everything you asked for in seniority order and when it goes into denial mode, it will deny your preferences in the reverse of how you bid based on your category position.

This is simplified, but generally correct and difficult to convey in a couple paragraphs.