Cynthia, The X stands for extension in both cases; there are other extensions besides these two, discussed in a Forum thread a couple of months ago. You have the Advanced Assembly Extension and the Behavioral Modeling Extension. You can get some more info on the PTC site, but they are not good at giving you a tabulated list of what detailed additional functionality is gained with each Extension (probably because that would also constitute a list of what they don't give you if you don't purchase one of the expensive Extensions). With Advanced Assembly, for example, you can use the full suite of Large Assembly Management tools such as Pro/Notebook (a.k.a. Layout) and Skeleton Models. David