Clarification on Electrical Generator Parameters

I've been looking at some generator parameters and I just want some clarification on what the following parameters represent:

a) Quadrature and direct axis reactance? I did a whole heap of googling on this, nothing made sense to me. Maybe someone can point me to a website/article etc. which explains this clearly?

b) Another query I have is about zero, positive and negative sequence impedances. I know about symmetrical components in regards to electrical faults, zero impedance = no angle between phasors, positive/negative sequence rotation of phasors a-b-c a-c-b etc.

Why are these measured impedances different on a generator? Is it due to varying currents and heating effects causing inductance and resistances of the windings to change under the various fault conditions?

c) I'm currently using SKM powertools to model an electrical system and for the generator, it asks for a paramater called If which alters the generators damage curve. Can someone tell me what If represents on a generator?

Thanks