Class A security was a classification for Starfleet Command messages in the 23rd century. These messages were typically scrambled and when a Starfleet starship received one of these messages, the communications officer was required to put it through a decoder.
In 2268, the USS Enterprise received a Class A security message. In the message, Starfleet Command informed the Enterprise that the Federation High Commissioner was en route to Troyius to attend the wedding between the Dohlman Elaan of Elas and the ruler of Troyius. (TOS: "Elaan of Troyius")