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Let suppose I create a hollow sphere, and want to extrude a sketch inside it limited to the sphere.
It appeared that the scketch is extruded in an arbitrary direction and I could not find how to switch the direction. I obviously tried with UCS inversed Z axis without success.
Here is after the extrude operation with "L" and selecting the inner sphere:
In this simple case, it does not make a huge difference, but when the "hollow sphere" become more complex, it is not always symmetric.