How is FID calculated in WPT

Assuming maxFID is the First input delay, please explain how this is being calculated and if it can be considered a valid representation of FID.
Alternatively what is maxFID, how is it calculated and what is its use.

I am aware of the importance of TBT, but if FID is also being generated, why is it not suitable to be presented and reported on as well?

Thanks in advance