.P21 files are files storing a three dimenional model format that can be used for three dimensional product data representation and exchange. The .P21 file is saved in a common ACSII text format that is specified by ISO 10303-21. .P21 files are used primarily for the automation of industrial systems and to help with integration purposes. NOTE: Express AIM (Application Integrated Model) and MIM (Modular Integrated Model) files must use the .STP extension.