An offline payment doesn't occur to the organizer through PayIt2's online checkout process. This is typically a check or cash paid in person or through the mail.
The organizer can choose to let payers mark themselves paid offline. This option is helpful when you (the organizer) wants to allow people to RSVP or indicate that they have sent a paper check or plan to pay you cash in person. The payer can simply mark themselves paid offline, fill out the registration questions and you can manually manage the collection process. Typically an organizer will add a note to the payer once they collect the offline payment to keep track of the details.