The Quick Answer
Adds pairings with a specified maximum value of credit ratio. Credit Ratio is defined as TAFB divided by the total credit for a particular pairing. The selectable range is based on the pairings for the particular bid package. The most efficient pairings contain lower values; least efficient pairings have a higher value of credit ratio with a maximum of 3.5 per the CBA.
The Manual
The higher the value of credit ratio, the less pairing credit for a given amount of TAFB.
Bidliner Insight
If you desire the most pay during your time away from base then treat this like a golf score, the smaller the number the better.