Thank you very much for your replies. I'm just learning Robert's Rules and didn't particularly understand the passage (above) in the abstract. I don't have a particular situation in mind. I think that's what may be causing the most problems for me in interpreting the text.
I think the ambiguity I see can be separated into two parts. 1) Does the 'essentially a new question' refer specifically and solely to the motion to commit?; and 2) How can debate itself ever make anything 'essentially a new question'?
1) I think it does refer specifically to the original motion to commit but I'm not entirely certain.
2) I'm more confused as to where the 'line' is. Say the main motion is for Xco. to purchase a new office building. There is a motion to commit to a special committee to study the matter. That motion to commit is defeated. Debate resumes. The motion to commit can be reconsidered at any time during the meeting until the criteria for the words of limitation ("until such time as progress in business or debate has been sufficient to make it essentially a new question") are met.
RONR says that the motion can be "reconsidered only until such time as progress in business OR debate has been sufficient to make it essentially a new question" (emphasis added). I don't see how debate alone can make anything 'essentially a new question', but RONR contemplates this scenario. As for the where the 'line' is, surely a debate that simply provides more information and clarifies the issue doesn't change the question, it would just change opinions on whether the motion ought to be adopted or defeated. Something more would seem to be required, but what does 'something more' look like?