"Invitational" usually means 10-12 high card points. A forcing 1NT response typically has a range of 6-12 points.
I consider it unwise to bid a forcing 1NT with only 6-7 points. Does the rule mean that "some" of my responses have to be in the 8-9 point range so that I don't "guarantee" the 10-12 point invitational range when I bid a forcing 1NT?