Each field can be broken up into sections to help identify where they are.
  • CNRelCt_1_01_Primary_Addressee

The "CNRelCT" because there are no spaces or "_"'s separating them is referencing the "Constituent Relationship Contact" while "Primary" let's us know it's the primary field inside "Constituent Relationships Contact", and "Addressee" lets us know which field is being exported.

Another example is "Cn_LastName", "Cn" let's us know it's "Constituent" only, with the field "LastName" being exported.