Simple.
"time_step" is a system-generated variable that gives the "expected" time to process this frame.
"time_frame" is a system-generated variable that gives the "actual" time to process the last frame.

My usage for time_frame was it you were trying to work out how far or fast something is travelling per frame,
then you should include "time_frame" in the equation to "smooth" out the differences between fast and slow frames.