A filename extension is the part of a filename that comes after the last dot character in the name. For example, if a file is named MyHabitat.spp, spp is the filename extension. Even if there are one or more other dots in the name, the part that comes after the last dot is the filename entension in Windows, Mac, and most other common consumer operating systems. For example, MyHabitat.hmo.spp and MyHabitat.inv.spp also have spp as their filename extensions.
For Windows, the filename extension is what tells the operating system which program is the default program for opening that file. This also determines the icon that appears in Windows Explorer. For .spp files, the default program should be SuperPoke! Pets Lite (SPP Lite). Conventionally, filename extensions are limited to only three characters since that was the limit for most Windows file systems used by consumers before Windows XP (2001). The three letter extension size limit sometimes causes problems because several programs on a computer may use the same extension. There are several programs other than SPP Lite that use the spp extension, the most popular of which is Serif PhotoPlus. PhotoPlus files are a totally different type of file than SPP Lite Habitat files and the two programs cannot open each other's files. A workaround for this conflict is to right-click and choose "Open with..." for the PhotoPlus files and leave the default program as SPP Lite. Due to the way that Habi Makeover works, SPP Lite must be the default program for opening files with the spp file extension.
By default, Windows hides the extensions for commonly used file types. Presumably, the designers of Windows thought that the filenames would look "friendlier" without the extension showing up, but this can sometimes cause confusion, since the user is not sure whether the extension is there or not. Also, this hidden extension function has been used to make a virus look like an innocent harmless file. For these reasons, some experts recommend that users stop Windows from hiding filename extensions.