One of the most powerful features of System Architect is the ability to tailor and extend the underlying metamodel of information stored in a project encyclopedia, by adding, hiding, or rekeying properties of definitions. This is done in a file called USRPROPS.TXT, which, when an encyclopedia is loaded, is parsed along with a file called SAPROPS.CFG (the main System Architect properties file). USRPROPS.TXT overrides SAPROPS.CFG. Properties are added, hidden, or rekeyed using a scripting language that is native to System Architect.

System Architect can be extended and customized in many other ways: