If there are 2 ABBRs, where does the system get each ABBR to calculate on?

Previously we had medical calculating on the ABBR in job and used Annual Rate to calculate Life and Disability premiums.  We recently had to change to use 2 ABBRs so we are populating the ABBR table either manually or with a custom process.

You cannot manually populate the ABBR table with PRIM since that comes from Job.  However the custom process we have populates either or both, depending on the ABBR type selected (BASE or PRIM).

To calculate, does the system always look at the ABBR table to determine the ABBR amount to calculate on?  Or does it still look at the Job table for PRIM, even though it is the ABBR table?