The weight rating is a measure of how much the mount can handle before vibration become excessive. This specification is dependent on the shape of the telescope. For example, if you have two telescopes that both weigh 15 lbs., but one is a Cassegrain with a very short tube and the other is a very long focal length refractor, the Cassegrain will be more stable due to the fact that the tube itself is much shorter. The extra length of the refractor tube acts as a "lever arm" and vibrations become much more prominent. So be careful as you approach the weight capacity limit that you don't overload the mount with a very long tube.