Split bottom bars for wax foundation, this is to give the wax room to "stretch, sag, droop" (what ever you want to call it). The grooved bottom bars will limit the movement of the wax in the downward direction and cause it to buckle in the middle, even it you cross wire it will buckle below the bottom wire or even pop off of the bottom wire.
Grooved for plastic foundation.
I buy from Brushy and Kelley, always satisfied with frames and foundation. But then again all I buy is split bottoms and hooked wire wax foundation, with the exception of thin surplus every now and then for comb honey.