What are the vertical lines on the top right portion of a return envelope?

Question

Often, one may find five or more vertical lines in the top right field of a return envelope - sometimes in other places. What is the purpose of the lines?

Answer 1

That's a FIM, or facing identification mark. It is used to help the USPS mail processing machinery get every letter oriented correctly. They need to separate stamped mail, metered mail, and business reply mail, and the FIM helps them do this.