Semi_rough is used to cut leftover from previous toolpath.
If you just know how to use UG & MasterCam those crap cam softwares, you will not
understand the magic of this option.even they added this option on newer version but they do not work good.
look at the jumping in UG roughing & unsmooth finishing toolpath(red one). what a joke! My company used UG to cut before but we dump it out from the Cam department two yeas ago.
Anyway, for designing UG still the best.