Matriculation, stated simply, is when you started working on your degree at Cal Poly. Pre-matriculation refers to courses taken before you started at Cal Poly.
For blended graduate students the matriculation date is when you started your B.S. degree. This would be your first quarter at Cal Poly as a freshman, or your first quarter after transferring from a different university. The matriculation date is not related to when you applied to the BMS program, when you were accepted, or when you changed your degree objective. The university considers the entire BMS program as a single program. Entering the blended program changes your degree objective but not your start date.
The matriculation date for M.S. students is more intuitive. It is the quarter they started the M.S. program. This is true for student who enroll at Cal Poly after completing their B.S. elsewhere, Cal Poly students who took a few years to work before returning for their M.S., and Cal Poly student who completed their B.S. degree and immediately started working on their M.S. degree.