That's a good one for Rudy.  He's the chemist in the group, but I'll stab at
it.

You can titrate to two different endpoints and determine total carbonate and

free carbonate using either two indicators or using a potentiometric
titration.

Proprietary developers in popular use today contain trace amounts of
potassium hydroxide.  If any hydroxide is in solution, it is impossible
to form bicarbonate.  When you don't have hydroxide, bicarbonate is formed
and is not effective as a developer.