Strokes that form an enclosed area do not have a rendered termination. In particular, all rectangle and border edges, as well as all 360-degree arc edges, are not considered to have any termination. Arcs with sweep angles less than 360 degrees and lines do have terminations at both endpoints.
Reference_Syntax.cap = “square | butt | round”
square (default)
The stroke terminates by rendering the end of the edge with a semi-circle at the edge’s endpoint, having a radius equal to one-half the edge’s thickness.
Form Model

cap