The ISPPBuiltins.iss file is part of Inno Setup Preprocessor and is automatically installed to the 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, so you can study it and reuse its techniques in your own scripts.