Currently on 2017 and use least squares for everything.
Regarding 2021+ My opinion is this new DEFAULT ..........'Constrained Least Squares' methodology, is not least squares, but is taking the highs and accounting for rocking conditions and they are calling it 'constrained least squares' when it is not least squares, it is the 'highs accounting for rocking'.
Correct. It does a least squared fit first, then shifts, external to material. I obviously can't share the exact algorithm on a public forum as it is Hexagon's intellectual property.