For instance, if you want a box that is always twice as long as it is wide, you can create a relation that would always keep this true. That way if you need a larger box, you would only have to change the width dimension, and the length dimension would automatically update itself.
Relations are very useful in complicated parts and assemblies.
They allow you to make large modifications without changing several dimensions. You can add several relations, one after the other. To end, just hit the return key on an empty line. When dimensions are created, they are given a symbolic name, such as d18 or sd9 . Relations are added in equation form.