Soil definition heights

[arnauft]

Hi guys

I am a bit confused by N1, Na and Nf when defining the soil in order to design footings. I am working with the following situation: a structure that goes 4 meters below ground level; below 4 meters, a 1.5 m RC column below it is defined in Robot. 

So, what values should I choose for N1 and Na? Are the ones in the screenshot below correct? Also, what is Nf?

Thank you

[arnauft]

I forgot to add that I would also like to know the structural difference between a pier and a RC column.