Hi Syabab,

Kindly find my response for your inquiries below:

1) Invitation Code

Is custom field that been used for registration process, where user that only have a valid invitation code (specified in field)
as example: https://take.ms/2imvB

2) Code Generator

This code generator will generated each of user that register into your site. With the unique code own by each user, for example site admin want to create an offline event as example a drawing competition. In the form organizer can create a field that competitor can input their own code without any hassle.
Thus, organizer can just check them via this code

Hope this helps