In various directories of your Nascar Racing sim you see files with a ".DAT" extension. In fact, these are bundles of files. With a utility like PACK/UNPACK you can unpack this bundle so you get a long list of small files. A patch, add-on or update replaces part of these small files to make the sim look or act differently. Next, the small files are packed back in the original bundle.
Say you have an update for your keyboard. It replaces the gray Enter key with a red one. Next you get another update, which replaces the Control, Alt and Enter keys with blue ones. From now on the special features of the red Enter key are gone; it's no longer on your keyboard. Makes sense?
When you apply a patch, and later on install another patch, it could very well be that part of the first patch is replaced by files from the second patch. In other words: the functionality of the first patch is (partially) gone. When you notice this, you could re-install the first patch and get its functionality back, but then you will find that the second one doesn't work anymore. Frustrating, but that's the way it works.
Bottom line: Always make a backup copy of the current ".DAT" file that is going to be modified by a patch. If you're happy with the changes, keep the new .DAT. If not, restore the old one.
And what if you have two patches that (partially) overwrite each other? Best thing to do is contact the author of either patch and ask him for advice. If you're lucky, he'll make a special version so you can use both patches.
Next page tells you all about the use of the PACK/UNPACK pair of tools.