The latin prefix "semi" literally means "half", not partly.
Semi-planing literally means "half-planning", like semi-annual coupon means a coupon every six months (exactly every half year, not sometime in the 1 to 364 day range).

Regardless the definition of planning and displacement, "semi-planing" is a rare event for the simple reason that it is rather difficult to sail at the exact speed that divides in two the speed range between displacement and planing speeds. "Pre-planing" is a more adequate expression, but that includes displacement speeds too.

A correct indication of the regime when a boat is sailing faster then displacement speed (whatever its definition) but slower then planing speed (idem) would be "post-displacement pre-planing regime".

Take care with those names, though: the existence of a precise name for something transmits a false sensation of scientific accuracy. A precise name lends no precision or accuracy to the phenomena itself. It also doesn't assure that it actually happens.