What's triggers an AI Touch sense "bump"?

I’m trying to figure out what exactly triggers this “touch” or “bump” to happen.

The AI Touch config setting gives you
the ability to detect when the AI
bumps into something or something
bumps into it. For example, in a
stealth based game, you may want a
Player to sneak by an enemy AI without
touching them. Using this Sense you
can determine when the Player touches
the AI and can respond with different
logic.