It’s not an area centroid point on irregular brep.
This is based on some observations and few experiments. I don’t have any knowledge of the the code.
For a curve or polycurve the curve start is used.
For a single surface the point at the mid u and v values is used as the origin. If the surface is trimmed the point may be off of the trimmed surface.
For a polysurface one of the individual surfaces must be selected. That surface is then used.
For a mesh a mesh face or edge must be selected. The mid-point of the face or start of the edge is used.
Or look at help: Rhinoceros Help
The Object option sets the construction plane in the active viewport to an object.
Select a curve, surface, or mesh face.
The origin and tangency of the new construction plane is based on the “midpoint” of the untrimmed version of the surface.
In the case of a mesh face, the plane goes to the center of the face and the face normal determines the construction plane z direction.
In the case of a circle or an arc, the origin will be placed at the center. The X-axis will point to the start point. The Y-axis will be 90 degrees away from the start point in the curve direction.