or weak (<26zp) with 6c M
or balanced 20-21hp
or semi-forcing in suit x (= 8 tricks)
Answers - undisturbed
If opener rebids 2NT, showing the strong balanced hand, Niemeyer can be started (3♣) when holding at least a 3card M. Indeed, the opener possibly holds a 5 card major.
A partner with 10+hps may show slem ambition , especially when holding a 5+ card.