On the XSI mailing list, a poster asked:

If I have a sphere and I want to get the point positions from the sphere and
see what their point positions are in reference to another null’s transforms
how would I do that?

It’s basically a two-step procedure. First convert the point positions from the local coordinate system of the sphere to global coordinates. Then, second, convert the global coordinates into the local coordinate system of the null.

It’s a little more efficient to combine the two matrix combinations, and then apply that to the point positions:

Here’s a video walkthrough of how to build an ICE tree that converts point positions from the local coordinate system (aka reference frame) of one object to the local coordinate system of a second object.