IOE Programmable Delay

Hello,

The data sheets for the FPGAs list the IOE programmable delays as follows (generic example):

Parameter: D1
Available Settings: n
Min Offset: 0
Fast Model: x ns
Slow Model: y ns

Could someone please shed some light on the Available Settings n parameter?
Does the delay setting need to be n all the time? Can it be less than that? If the delay setting is 1, what is the effective delay?

Thank you!