is boundary of
is 2D boundary of
a relation between a 2D immaterial entity (the boundary) and a material entity, in which the boundary delimits the material entity
A 2D boundary may have holes and gaps, but it must be a single connected entity, not an aggregate of several disconnected parts.
Although the boundary is two-dimensional, it exists in three-dimensional space and thus has a 3D shape.
the surface of my skin is a 2D boundary of my body