Boundary
The set of points that define the limit of a spatial object:

Points, multipoints, and geometrycollections do not have boundaries.

The boundary of a linestring is a multipoint object. This object contains its start and end points.

The boundary of a multilinestring is a multipoint object. This object contains the start and end points of all the linestrings that make up the multilinestring.

The boundary of a polygon is a linestring that begins and ends at the same point. If the polygon has one or more holes, the boundary is a multilinestring that contains the boundaries of the exterior polygon and any interior polygons.

The boundary of a multipolygon is a multilinestring that contains the boundaries of all the polygons that make up the multipolygon.