Thanks to Crantisz I think I finally understand:
The nearest face is the face built on an edge that is the nearest to the Source Position.
- not a face with the nearest center to s.p.
- not a face with the nearest vertex to s.p.
- not a face that could have a point on its surface closer to the s.p. than any possible point on any other face.
But what is the nearest edge then?
The nearest edge is an edge, on which you could place a point closer to the s.p. than any point you could place on any other edge.
- not an edge with the nearest center to s.p.
- not an edge with the nearest vertex to s.p.
Below I spawn an icosphere on the nearest edge to vertex #0:
Below I spawn an icosphere on the nearest face to vertex #0 (I positioned vert #0 to lay on the surface of the big face):
Now I stretch and move the small face along X, so neither its vertices nor centers of its edges are closer to s.p. than the top vertex (#2) of the big face) - and yet it still spawns the icosphere, because the top and bottom edges come closer to vert #0 than any other edge: