What's the difference between "Full controller support" and "Partial controller support?"

Within Steam's "Big Picture Mode," games are shown with a half- or full-controller to signify partial- or full-controller support, respectively.

However, many of the games listed as "partial controller support" I've played for hours with a gamepad, without ever needing to use a mouse or keyboard. These include

• Vessel
• Prototype 1+2
• Batman: Arkham Asylum and Arkham City
• Beat Hazard
• LA Noire
• The Last Remnant
• Bastion

And many others. So, what exactly determines if a game has full- or only partial-controller support?

The Big Picture Release Sale page on the steam website has an answer. Here's a screenshot of the right hand side from that URL:

And here's the text for completeness:

Full Controller Support

Games with Full Controller Support will work without a mouse and keyboard on hand, from installation to play and beyond.

Partial Controller Support

Games with Partial Controller Support may require a keyboard and mouse from time to time, during installation or for limited interactions.

So, basically, it would appear that a game with full support never needs input from anything that isn't a controller, whereas games with partial support can mostly be played with a controller but you'll probably need a keyboard and/or mouse to do something at some point (which might just be to click 'OK' on a window to install DirectX or enable controller in options on the first run or maybe click 'Play' in a launcher every run, etc).

Partial controller is also for games with type chat. (like black ops 2, or far cry 3) it just says partial because you need the keyboard to type.