Forces in the machine/workpiece frame of reference:
This means that the forces as seen by the machine structure beyond the spindle bearings, the workpiece and workholding fixture are essentially constant in time. That can be beneficial if the relevant flexibility is on this side (v-wheels and belt drive, I’m looking at you).
(This is an unequal-helix endmill, hence it can’t be completely flat).
Right-click and select “tool coordinates”:
Now these are the forces in the tool’s rotating frame of reference, so a flat line transforms into a phase-shifted sine. This is the force that is exciting the rotating part, endmill and spindle shaft, so this view is relevant for a setup where the relevant flexibility is there, e.g. when using a small-diameter tool or an anorectic spindle shaft.