3.B in the contract describes it. It's 3 pages long so it's easier if you just read it yourself.

As a synopsis, they calculate how many pilots are required to staff the fleets (using metrics like historic pilot-to-block hour ratios etc), and everyone senior to that number gets the C band pay regardless of the plane type they fly.
So more 175s = more pilots required to staff the fleet = the C band goes more junior.

July 1st the seniority cutoff was 504. Next update is October 1st.
We only have B and C bands, because A band was 60-70 seat Turboprop CA scale.