Apparently, the last digit of the fare basis code encodes the bundle/benefits you're eligible for.

This is the information I have at the moment:

Fares basis codes ending with 1 are "basic" fares published in the GDS.
Fares basis codes ending with 2 are "Essential" fares.
Fares basis codes ending with 5 are published in the GDS.
Fares basis codes ending with 7 are "Plus" fares eligible for free standby on an earlier flight.
Fares basis codes ending with 8 are eligible for free standby on an earlier flight.
Fares basis codes ending with 9 are eligible for free standby on an earlier flight.