Dear @prabath there is a geometrical background you have to understand:
As soon as the gausian curvature is not 0 for the entire surface, it is not developable.
As physical materials like cloth, rubber and even steel-plates can be compressed / expanded while they are formed, it makes sense, that CAD / rhino offers a function to approximates the 2d / flat representation that can be cut and later deformed.
_squish is as far as i understand the most advanced command regarding those approximations.
_unrollSrf makes sense, only if the surfaces is developable
_unrollSrfUV will allow you to keep the UV-domain of the flat and curved version and also is only useful with developable surfaces
_smash is an older version of _squish
what do you need the 2d-shapes for ?
cutting some cloth / leather / sheet metal ?
it might be good, to split the parts in the middle of the curvy section - as the straight section is similar to a developable cone.
you might want to check for tutorials like this - or similar: