Simple "Plus X" Method
You would use this method when you 1) need limited information on guests of registrants and 2) the guests of registrants will be doing the same things (activities) as the main registration.
You are inviting your main registrant to sign up for to go see a baseball game and they are allowed to choose how many are in their party. You just need to know the names of their guest.
You are hosting a golf tournament and selling a golf foursome where you simply need the name of the other three members of the foursome.
You are hosting a seminar and the registrant is purchasing seats for the seminar.
Full Registration Method
You would use this method when you 1) need full detail on guests of registrants and 2) the guests of registrants can chose different registration items than the main registrant.
You are holding a conference and conference attendees can bring guests. A typical day includes a general session in the morning and elective activities in the afternoon. The guest of the attendee can chose different (or same) activities as the attendee for the afternoon.
You are holding a simple event but you want to collect a full set of information (e.g., name, address, email, phone, membership status, etc) on all people attending the event.
Bulk Registration Method
You would use this method when you want to allow one person (who is not attending) to purchase multiple registration slots and complete those registrations or invite others to complete those registrations.
You are an archery coach and you want to register your students for a tournament. The coach can purchase multiple registrations and then register or invite each student.
You are a training manager and you are registering multiple people for a training class.