What is "letters missive"

Missive \Mis"sive\, a. [See Missive, n.]

1. Specially sent; intended or prepared to be sent; as, a letter missive.
   --Ayliffe.

2. Missile. ``The missive weapons fly.''
   --Dryden.

   Letters missive, letters conveying the permission, comand, or advice of a superior authority, as a sovereign. They are addressed and sent to some certain person or persons, and are distinguished from letters patent, which are addressed to the public.