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Under the "Site" Menu is the command "Edit Sites". Once selected you get a window many are familiar with, the site definition window. From this window, towards the bottom of the column of buttons is an "Export..." Button and an "Import..." Button. The export button allows you to export a Site's Definition. You can then archive it, e-mail it, share it, etc. Later when you have lost it, had to reinstall DW MX, or anything else you can simply use the "Import..." button to reinstall your Site's Definitions. If you work on a lot of sites this can be very important.
As a rule of thumb, in my office, I make everyone how fills out a new site definition export it and back it up to a file server as soon as it is complete. This has been a life saver for us when we loose them, need to share them with each other, or if we want to work on a site from a different machine than normal.
Don't go through needless head aches. This feature was added for a good reason. Don't let it go unused.