Flight Files (FSX)
From FSDeveloper Wiki
Placeholder for content and structure of Flight Files (.FLT).
This document describes the format of flight (.FLT) files, which are used when a session is saved off, often to be loaded again later on.
Flight files can be amended after they are saved off and before reloading, or applications can be developed that output flight files in the correct format that can then be used as input to the simulation.
When a flight file is saved off, two (or three, if a mission is being saved) other files are saved off also, with the same filename but different extension.
If the .WX weather file is not present, the simulation will simply use default weather.
An application that creates flight files to initiate the simulation in a certain state
Creating just a flight file is simpler, but the mission system does add more options -- such as the easy addition of objects to the simulated world.
MSDN - ESP - Flight Files - Details their structure & content.