BlockingVolume
A Blocking Volume is a Brush that only provides collision. It replaces UT's Invisible Collision Hull, which was prone to semisolid errors.
It's basically an invisible brush that is there for the only purpose of blocking actors from moving into a space. These can be used for complex objects where you may want to make them nonsolid, but still block movement (for map efficiency reasons), or if there's a space that a player could become trapped in so you do what's best for them and keep them out of it (unless you decide to make said space a killzone, always good for a couple laughs 
).
Collision
The settings for what the BV blocks are under Actor/Collision.
Related Topics
Highlander: Re:Blocking Projectiles. Try setting bBlockZeroExtendTraces or bBlockNonZeroExtentTraces to true or fiddle with the settings under collision. (Not at home system so cant check)
