So what does polygon mean?

[LeeJaeYoung]

API_Polygon | ARCHICAD API (graphisoft.com)

• the first coordinate of the array is not used; must be initialized to (0.0, 0.0) for polygons, and to (-1.0, 0.0) for polylines,

Q1. I'm curious what the polygons here mean.
At first I thought you meant a closed polyline.
However, even if initialized to -1.0,0, the polyline was closed.
So what does polygon mean?

[BerndSchwarzenbacher]

The open polyline online works if you are creating a API_PolyLineType Element.
Other element types with polygon information can only have closed polygons.