Topeak's MTX beam racks are a very useful accessory for your bike, but as they fit onto your seatpost they don't always leave much room to fit the other essentials, like a clip on, mountain bike style mudguard.
That's why Topeak decided to produce a clip on mudguard specificly to fit their MTX and EX beam racks.

It's designed and manufactured by Topeak to go with the Topeak racks, so if you have the rack you know this will work well.
Easily clips on to the rack to help keep the mud off your back and your rackpack, and save space and reduce muddled untidiness on the seatpost compared to a standard, seperate rear mudguard.