Associated nets are usually used for length matching. You can associate an array of nets joined by discrete components, creating an associated net, to which you can apply rules as you would to a single net. The length of an associated net is the combined lengths of the nets and discrete components of which it is composed.
A search for associated net in the Layout User's Guide will bring up information on creating and using associated nets.
On the same subject, I have successfully created a 'group' of matched length 'Associated Nets' as suggested by first creating the individual nets to be in an 'Associated Net' and then in 'Associated Net Rules' selecting which Associated Nets to be included in a Group, then selecting the 'High Speed' button to set up the 'Matched Lengths'.
However, I also need to have nets which are not Associated Nets included in this same Matched Length group.
I have tried to create a new class and add Associtaed Nets into this class, but I can't seem to figure out how to do it.
Bottom line: How can you have a Matched Length Group consisting of both Associated and non-Associated Nets in ther same class (or even group)?