The XTD folder contains a number of files that you may not want to access all the time. Files that you use a lot like air.ini, or buttons, are contained in the STD folder.

Use the XTD folder to put things that clutter up your STD folder, and for modifications to your GUI (the game menu screens, and background images for the main game)

This folder also contains a "placeholder" file, which is used to create divisions in you air ini file (if you dont know what this means, dont worry, eventualy it will all be less confusing!)This placeholder is a "Hashed" file, and it's name is CB4787E2ABCAFD90
Dont delete it!

The "Presets" folder in here, is a patch that will ensure that, if all else fails, you will at least have default engine sounds in your game. Leave it as it is, and install new sound modifications in your mods folder, in folders named anything that will make them be placed IN FRONT of your XTD folder. (they wont work in your files folder while this presets folder is present here)