That is the reference frame, or reference datum of your local coordinates. It explains what point on the map the corrections will be aligned with.
The F9P uses RTCM3 corrections, it is the data format for the corrections.
For example my local reference frame is NAD83, but they are conveyed to me over NTRIP in RTCM3 format.
I can also use WGS84 reference frame, conveyed over NTRIP in RTCM3 format
The absolute position on the ground of the same coordinate numbers will be different by many meters difference between the two reference frames. This is why falling back to WAAS is a really bad idea, because you can never sync to the same reference because its always on the move with time, so you get a position jump.