Another thing to add for the next edition....
From what I can determine, the best definition for 'Soft-in-plane' refers to a rotor head that has an elastomeric bearing to permit the blade to lead and lag, but not as much as it would if it were a fully articulated rotor. As far as I know, only Bell uses the term (but I'm probably wrong), and they developed the term because the rotor head to which it first referred- the Bell 412, was very different than the 212.
The 212, as a teetering rotor obviously did not permit in-plane motion of the blade as it rotated, whereas the 412 with the elastomeric bearings did permit the blade to lead and lag as it whirled around.