[Leica] A question about lens focus

[abridge at gmail.com (Adam Bridge)]

When a lens is focused at, for purposes of example, 3 meters, is the
focus for a plane that runs perpendicular to the centerline of the
lens measured from some point (like the film plane) or is it a
spherical surface measured in the same way?

In my head I can make this work either way but I would like to know
how it really works.

Thanks

Adam