The type of transitions that occur at the beginning and ending of a toolpath depend on whether that portion of the toolpath is a loop or an open toolpath. The transitions for open toolpaths are linear and are controlled by Lead in angle and Lead distance. Transitions between closed toolpaths are arcs. See Default ramping for milled finish passes for more information on the parameters controlling closed loop toolpaths.
The Lead-in angle applies only over the Lead-in distance, so if the Lead-in distance is 0, the Lead-in angle has no effect.