Round slots have two rounded ends, each having up to 180 degrees of measurement radius. The value in the Meas Ang box defines how much of the radius to measure. PC-DMIS takes the input value and divides it in two, measuring half on each side of the slot angle vector.
Suppose you type 90 in this box. When creating the Round Slot, PC-DMIS would then measure 45 degrees to the left, and 45 degrees to the right of the angle vector (or vice versa depending on the angle vector).