An airmail etiquette, often shortened to just etiquette, is a label used to indicate that a letter is to be sent by airmail

Airmail

Airmail is mail that is transported by aircraft. It typically arrives more quickly than surface mail, and usually costs more to send...

. The term is from French étiquette "label, sticker" (cognate to stick), from which also English etiquette

Etiquette

Etiquette is a code of behavior that delineates expectations for social behavior according to contemporary conventional norms within a society, social class, or group...

 "rules of behavior".
Because the etiquettes are just instructions to postal clerks, and have no monetary value, their printing and distribution need not be as carefully controlled as for postage stamp

Postage stamp

A postage stamp is a small piece of paper that is purchased and displayed on an item of mail as evidence of payment of postage. Typically, stamps are made from special paper, with a national designation and denomination on the face, and a gum adhesive on the reverse side...

s, and most are privately produced.