The ISPPBuiltins.iss file is part of Inno Setup Preprocessor and is automatically installed to root of your Inno Setup folder. This file is automatically included at the start of preprocessing: some of ISPP's functions are actually implemented by this file instead of being built-in. It also contains common declarations such as special constants. The file is a regular Inno Setup Script file so you can use all of its techniques in your own script as well.