Unity Car Tutorial Sounds - what does DEFKL mean?

So I've tried Unity's tutorial on car physics (https://www.assetstore.unity3d.com/en/#!/content/10) and it's pretty good.
I've managed to adapt it's sound controller to my needs, but a question still comes up in my mind.
What does D-E-F-K-L mean? Is it some engine-sound-design theory thing or is it just random? I kind of get what each of them is used for, by sound file names, but still... does it have any specific reason to me named in that way or is it just arbitrary?

Cheers!