setUserMatrix explanation

Hello, I am trying to use the setUserMatrix method found here Prop3D | vtk.js

I would appreciate some clarification on what this parameter is actually doing when you change its mat4 values. Just something simple, like what do the first 4, second 4, etc. transform.

I tried experimenting with the values and felt more confused than when I started. I’m attempting to use it to rotate a volume by some number of degrees ideally

The user matrix is the first transform applied to an actor’s transform. AFAIK it should be a homogeneous matrix, with the top left 3x3 being a scale+rotation and the right 4x1 column being your xyz translation with a last component of 1.