I think that is collide. For example I think below it means that those things will collide with the bars (player (movement), club (melee), rocket (rocket)
<block name="ironBarsCentered">
<code>
<property name="Collide" value="movement,melee,rocket"/>
<more code>
</block>
In this one it will also stop bullets and arrows.
<block name="glassCTRSheet">
<even more code>
<property name="Collide" value="movement,melee,bullet,arrow,rocket"/>
<last code I promise>
</block>
I am sure someone can confirm or deny this .