Almost every Windows program will create files that it alone is able to open. For example, Winplot creates files that are identified by their extensions .wp2 and .wp3. Until your installation of Windows is made aware of this fact, however, these files are all unknown, and their assigned icons are generic “flying windows”. When you double-click such an icon, you would like the file to open automatically, but it will not. Instead, the “Open With” dialog box appears. Windows is asking you what program you want to use to open the double-clicked file.
This is your opportunity to “register” a (new) file extension with Windows. It is a simple process. If the intended program is not already in the list, click the “Other...” button, and use the File Open dialog box to find the program. Select it (click its icon once), then click Ok. The program icon will appear (selected) in the the list of known programs. Check the “always use this program” box (so you will not have to do this again with this type of file), and click Ok one more time. The selected program will try to open the file you double-clicked in the first place.
Unless you chose an incorrect program, the file will open. Because you checked the “always use this program” box, all files with the same extension will now be assigned to the chosen program, their icons will be changed to the icon that belongs to the program, and they will all respond the way you want when you double-click their icons! This is what it means to “register a file extension” with Windows.
Once the file extension .wp2 is registered with Windows — probably by assigning it to Winplot — the Winplot program icon appears in the list of known programs. If you double-click a file of type .wp3, and if this file type is still unknown to Windows, you will again get the “Open With” dialog, but now the Winplot icon will be in the list (scroll down to find it, instead of clicking “Other...”).
The overall management of registered programs and files is done by clicking View|Folder Types in the “My Computer” menu. This is where you would go to unregister a file type, for example.
An experienced Peanut user describes a registration problem and how it was solved.