How does the STBuffer function measure distances around a polygon?

I am looking at the expression:

select SP_GEOMETRY.STBuffer(100)

and wondering how the 100-meter buffer is calculated around the geometry of polygons.

e.g., is it from the centre of the polygon? or does the buffer start from the edges of the polygon?

I am struggling to find helpful documentation on this so if you have any links let me know!

I am wondering because the function is trying to pick up boundaries which are within 100m from each other, but when running the function, not all the properties within 100m are being selected.