Thanks Mentor.
Follow up on 1. Please explain AxB keeping this in perspective. Are you referring to a feature cross?
Lets say A = [1,2,…10] - 10 distinct values
Lets say B = [1,2,…10] - 10 distinct values
Thus AxB = 100 possible values
In the training set, lets say A & B appear with a pattern
A,B
1,1
2,2
1,2
2,1
3,3
4,4
5,5
(much less than all of the possible values of AxB which is 100)
But the serving set shows all 100 possible values.
Hope this helps clarify.