when setting cplane to object, what, if anything, determine the plane’s z axis?

i don’t see anything in the docs.

when setting cplane to object, what, if anything, determine the plane’s z axis?

i don’t see anything in the docs.

It would be related to how the CPlane’s x and y direction are seleted. Rhino uses right hand coordinte system which means if x and y are specified then z is also determined.

If a CPlane is set to a surface then (based on observation) the CPlane at its origin is tangent to the surface. I’m not sure how the location of CPlane origin is selected but it may be at point on the surface corresponding to the mid-values of u and v. The CPlane x axis is tangent to the u direction of the surface at the origin and the y axis is in the tangent plane and normal to x. Positive x in the same direction as increasing u. There are two choices for positive y and the one more closely aligned with the direction increasing v at the origin is selected. The z axis is normal to the x axis and y axis (and also the tangent plane at the origin). Positive z is based on the right hand rule and the direction of postive x and positive y.