What and why are some mail pieces tabbed?

Tabbing is required for a folded self-mailer to be consider "Automation Compatible." The tabbing process assures the post office that your bar-coded mail can be processed by machinery and will help ensure that the mail piece arrives at its destination in better shape. When designing a self-mailer, final folds should be on the far right and bottom edges of the mail piece. Otherwise, two tabs will be required. Self-mailers with folds at the top edge increase the chances of getting jammed in the mail processing equipment at the post office. Paper weight also affects the tabbing requirement for self-mailers.