This enables a relax operation that works only on face sets boundaries,
which smooths the jagged edges that are produced when painting or
expanding face sets by sliding the topology without affecting the shape
of the mesh. This has many uses in hard surface sculpting for things
like sculpting panels or smoothing surfaces. It can also help when
working with remeshed topology as it makes the face sets looks better
and more organized if needed.

The operation is implemented as an Shift smooth in the Draw Face sets
tool, similar to the Slide/Relax tool

The same operation is also available in the mesh filter to smooth all
the face sets boundaries uniformly or to smooth the face set under the
cursor with the Use Face Sets option.