You can read the code for boundary(). It creates an alpha shape object with the vertices and infinite alpha. It then uses the alphaSpectrum() method on the alpha shape object:
"a = alphaSpectrum(shp) returns the values of the alpha radius that produce distinct alpha shapes. a is in descending sorted order. Each element in a represents a value of the alpha radius that results in a distinct shape."
It then goes 1-S of the way through the list and uses that alpha.
