It is an executable rule file package generated by ESRI's product ArcGIS CityEngine software. This file package can perform built-in functions in CityEngine itself and its third-party software. It is mainly used to generate 3D models of cities and their buildings. This rules file is developed by CGA language.A rule package (*.rpk) is a compressed package, containing compiled CGA rule files (*.cgb), plus all needed referenced assets and data. Rule packages are an easy way to share procedural rules in a handy single file. For finding the package content, the rules are analyzed.