Interactive Exchange (IFX) is a de facto standard for Web-based billing systems, comparable to Open Financial Exchange
(OFX). Unlike OFX, which formats transactions and invoices in Standardized Generalized Markup Language (SGML), IFX implements all specifications and Extensible Markup Language (XML). XML defines custom markup languages for several classes of documents, such as invoices, order forms, or product catalogs. It can do this because XML is implemented in Standardized Generalized Markup Language (SGML).