confusion with merge and field codes

[BluRay]

Hey,

I will like to know the difference between merge and field codes. Or are they same?

Also help me to understand this questions:
-- Save this document showing the merge and field codes as (Filename) in a format that keeps the merge code information.

How do i save "in a format that keeps the merge code information".

Also, in another question it says to print the document showing merge and field codes. I have attached two .pdf documents. Please tell me which is the correct one.