Editing your contact fields
Go to the label Contacts and click in Fields, then click in the field you want to edit.
You can change the name of the field, the type between text, number, date, true or false and decimal.
In Advance Settings you can change the Key and the sort. The Key is the code you have to write to customize your emails and always has to be between $$ and without spaces, uppercases or special characters, for example $lastname$.
Sort is the importance of a field to order them, being 1 the most important. The fields Email and Phone can't be edited or sorted.