I've never been clear on what the limits on nesting are.

Specifically, I actually thought that it was confined to a single airline/alliance. Obviously, that appears to not be the case.

So a second question, does anyone know what the destination limitations are?

For example, would flying from LHR back to any point in the US (or, say, Asia) be considered nesting? (for example, if I flew SFO-LHR-SFO with LHR-MIA-LHR inside it? What if I flew LHR-LAX-LHR inside it?)

Steve