A little bit of research, as I am not a survryor:
The P.I. on these first two markers stand for Point of Intersection. The PI is where two straight boundary or centerline lines (called "tangents") meet. These lines are often used to define the boundaries of a road, easement, or larger parcel. The surveyor marks this point to define the geometry of the survey.
the 99151 is an identfying number assoiciated with a specific point. It's a unique identifier assigned by the surveyor for record-keeping. This number allows the PI marker to be referenced in the official survey documents, such as a plat map or legal description. The map would detail the exact coordinates of point "99151" and its relationship to other markers.
If I am wrong someone with some knowledge please correct me.
Thanks