What file/program types are classified as "Other" category in iTunes

What file types or program types add to the total device GB capacity count for the "Other" category under the usage bar in iTunes?

It is that color bar at the bottom of the device screen in iTunes when you have your iPhone or iPad plugged in and are syncing with it. Audio, Photos, Apps, Other, ...

My "Others" bar shows 6.5GB usage but I only have audio and some apps on my iPhone5. It seems like this is wasted space that is taking away from my total phone storage capacity.

I also looked on my phone General=>Usage=>list of files... to confirm nothing is hidden.

Advice?