ARINC 661 is a free software that uses the ARINC standard in order to define the interface between the CDS and Aircraft avionics systems.
ARINC 661 standardizes the runtime communication between the CDS (Cockpit Display System) and the UAs (User Applications).
ARINC 661 also provides a way to completely define GUIs, using standardized configuration files.
An ARINC 661 compatible CDS must have a Server capable of creating the GUI hierarchy during initialization, thus avoiding the need to be recompiled if the GUI definition changes. ARINC 661 was originally created with the expectation of being used on Air Transport aircraft only.
Thus the feature set and overall design are conservative. Today ARINC 661 is viewed as being useful for Cockpit Displays on Air Transport, Business/Regional and Military aircraft.
ARINC 661 development often happens in a certification context (RTCA DO-178B/EuroCAE ED-12B), and this fact has influenced the contents of the standard.