This variable is global in scope and is used to hold a DS map when used in the Asynchronous Events, and -1 at all other times. The actual contents of the DS map will depend on the type of asynchronous event that it is populated in, and the function that was used to trigger the event, so refer to the individual pages for those events.
Real (integer value)
if ds_map_find_value(async_load, "id") == back
if ds_map_find_value(async_load, "status") >= 0
background_index = back
The above code is taken from the Image Loaded Asynchronous Event and will first check the ID of the async_load DS map that has been populated, then check the status of the callback. If the value is greater than or equal to 0 (signaling success) the result from the callback will then be used to set the background index to the newly loaded image.