The cap style describes the way that line symbol layers will terminate when combined with a geometry. The term "cap" refers to the end of the line. The options here control the shape that cap will take.
The StrokeSymbolLayerCapStyle can be one of:
(0) Applying the "butt" cap style will terminate the line ending exactly where the geometry ends. The end cap will be squared off. That is, two 90 degree angles form a butted ending to the symbol. In dynamic rendering mode, caps are always rendered with the
(1) Applying the "round" cap style will terminate the line ending with a semicircle of radius equal to the stroke width. That semicircle will be centered at the line endpoint.
(2) Applying the "square" cap style will extend the termination of the line ending past the end of the geometry. The end cap will be squared off. That is, two 90 degree angles form a squared ending to the symbol.