All geometries occupy a position in space defined by their interior, boundary, and exterior. The exterior of a geometry is all space not occupied by the geometry. The boundary of a geometry serves as the interface between its interior and exterior. The interior is the space occupied by the geometry. Subclasses inherit the interior and exterior properties directly, yet the boundary property differs for each.
The ST_Boundary function takes a geometry and returns a geometry that represents the source geometry's boundary. For more information, see ST_Boundary.