What does Cut Boundary Points in Edge Cusp node do? It isn't mentioned in documentation and it does not seem to do anything.

if the edge in group is touching the boundary of the geo, the boundary point is not split by default
so you have to check this option

like when you do a grid and select one row in the middle to cusp, you will see the difference
to see it more you can move primitives of one cusped half and you should see it not fully split unless this option is checked