What is "VDF" in the documentation?

Is it an acronym covering any type of the supported files? Or is it a file format, that I have never heard of? I don't find much when I google for fx VDF VirtualBox. 

In the last section it says: "There have been issues mounting VDFs writeable. Mounting them for writing seems not to be reliable". So is that true for all of the file formats; vdh, vdi, vmdk, raw? 