The AV-Protocol was originally defined to enable Venus, the desktop part of the Gemini replacement desktop, to communicate with desktop accessories which explains its name: AV, short for Accessory<>Venus.

The multitasking boom extended the usefulness of the AV-Protocol because parallel running applications could use it to talk to each other.

Not all AV-Servers are created equal. The AV-Protocol defines over 20 standard messages and replies yet some so-called AV-Servers only support a few messages - which obviously limits their usefulness. Typically the AV-Server sends VA messages to AV-Clients and they reply with an AV message.

The most commonly supported message is VA_START and is used to launch programs or pass parameters to parallel running applications - typically to display files.

Most of the other messages are concerned with window and file management. GEM windows can be opened, shut, updated, have their font changed and accept data Drag&Dropped from windows belonging to other applications. Files can be moved, copied, deleted and have their details examined.