What does the term "flat" refer to in USPS mailing categories?

The term "flat" in USPS mailing categories specifically refers to large envelopes that are rectangular and flexible. This classification encompasses items such as documents, magazines, and catalogues that exceed certain dimensions but are not rigid. Flats are designed to be lightweight yet larger than standard letters, providing an efficient way to send materials that need to maintain a flat shape during transit.

This categorization benefits both senders and postal services by streamlining the sorting and delivery process for these types of items. It helps ensure that materials like magazines can be sent without being folded or damaged, which is crucial for preserving the quality of the content inside. Understanding this classification is essential for navigating mailing options effectively and choosing the right category for specific items.