FBtri = freeBoundary(TR)
returns the free boundary facets of TR.ConnectivityList.
A facet in TR is on the free boundary if it is
referenced by only one triangle or tetrahedron.

[FBtri,FBpoints] = freeBoundary(TR) also
returns a matrix containing the vertices of the free boundary facets.

The vertex IDs in FBtri reference
a specific matrix, depending on the syntax you choose:

If you call freeBoundary with one
output argument, then the elements of FBtri are
row numbers of TR.Points.

If you call freeBoundary with two
output arguments, then the elements of FBtri are
row numbers of FBpoints.

FBpoints

Free boundary points, returned as a matrix containing the coordinates
of the vertices of the free boundary facets. Each row of FBpoints contains
coordinates of a vertex.

Definitions

Vertex ID

A row number of the matrix, TR.Points or FBpoints.
Use this ID to refer a specific vertex in the triangulation.