This event can only be triggered by the callbacks from various specific Social Functions and Xbox Live Functions and will return a DS Map stored in the variable async_load, containing different key/value pairs depending on the callback from the function that has triggered the event.
This event may also be triggered by Video Playback.
The DS map will contain a number of keys, the most important of which is the "id" key. This will return a constant which can then be checked in code to determine which of the many callbacks the event has received. Once you have parsed this key and compared the value returned with the available constants, you can then continue to extract the rest of the information from the map.
The function pages on Achievements and Leaderboards in the Reference section detail the functions and callbacks as well as the constants used to trigger this event, but there are also certain circumstances in which it will be triggered without a function call from the game that is running:
Any of the above callbacks will trigger the Social Event and an async_load map will be generated with the following details (note that there is a different "id" key value to define each of the different reasons that the event was called, but all other contents of the map are the same):