There are 27 different types of top level messages defined in ITU 1371-4 (out of a possibility of 64) that can be sent by AIS transceivers.

A full list can be downloaded from the download section on the right.

AIS messages 6, 8, 25, and 26 provide "Application Specific Messages" (ASM), that allow "competent authorities" to define additional AIS message sub-types. There are addressed (ABM) and broadcast (BBM) variants of the message.

Addressed messages, while containing a destination MMSI, are not private and may be decoded by any receiver. One of the first uses of ASM's was the Saint Lawrence Seaway use of AIS binary messages (message type 8) to provide information about water levels, lock orders, and weather. The Panama Canal uses AIS type 8 messages to provide information about rain along the canal and wind in the locks. In 2010, International Maritime Organization issued Circular 289 that defines the next iteration of ASM's for type 6 and 8 messages.