HEM is a relatively new term.
It means High Efficiency Milling. It only became available when constant tool engagement toolpahs became almost standard on most of the CAM software.
Unlike HSM that utilizes chip thinning effect, HEM relies on much larger widths of cut and thus chip thinning does not occur. What gives it its name is much higher material removal rate that would normally be possible.
When you are machining a pocket you are most often only milling at about 50% WOC. But nevertheless you need to calculate speeds and feeds based on the fact that the very first move and every corner will be full slotting action. Which means that the whole pocket needs to be machined at lower feedrate.
HEM uses constant engagement toolpths to make sure that this never happens and that Width of Cut remains optimal. Tool never needs to make a full slot so you can ramp up the feedrate as if you were doing outside profiling.
Here is a video of a 1/2" 3 flute endmill machining a 5/8" deep pocket in aluminum at full depth. Normally this pocket would have been machined in 2 steps at 150 inches per minute.
Using Constant Tool Engagement toolpaths we can go full depth at 0.175" stepover and 275 inches per minute.
The advantage of this method is obvious- Higher Productivity.
HEM is not ideal for all cases and each application merits its own method of machining, but its always nice to know more than one way to do your job.