As a follow-up to the last note, here's a simplistic view of part of the project management triangle (remember: On Time, To Budget, Right Quality -- pick any two).
Sometimes scope is drawn in explicitly (a tetrahedron!) sometimes not. Sometimes Quality = Performance/Requirements, other times Requirements have their own dimension. Basically, it gets more potentially confusing the more properly you think about it, and despite many brave attempts the Triangle is a persistent idea. The central idea of a consciously managed trade-off -- along any side of the triangle -- is the one thing all the models have in common.
Let me have a go. This one's called Risk vs Scope :