Any mailable matter may be mailed as first class mail.
First class is used for normal correspondence.
The advantages of first class are faster delivery times, free forwarding or return of undeliverable items, and first class mail may be sealed against inspection. All first class mail exceeding 13 ounces is eligible to mailed priority. Priority mail is best known as the 2-day service. Delivery averages 2-3 days. Special priority packaging is available from mail services for mailing priority letters and packages.