Boundaries > Subdivide Boundary marks specific vertices of a polygon object so they are not affected by editing operations such as Edit > Remove Boundary and others, and so that they cannot serve as boundary endpoints in Boundaries > Edit and Boundaries > Relax.
The parameter is:
Snap to Boundary (checkbox) - specifies whether each mouse click is interpreted as a click on the nearest boundary line or the nearest polygon vertex. Typically, leave this box checked.
To subdivide a boundary, click Edit > Boundary > Subdivide Boundary, click individual vertices on the polygon object (usually, vertices that are traversed by a boundary line), and click OK.
The subdivision points exist even after the boundary is removed. To remove the subdivision points, see Boundaries > Clear Subdivision Points.