XTension developers are individuals or companies whom develop, manage and publish XTensions for QuarkXPress, a leading desktop publishing(DTP) tool for publishers designers and printers, which is published by Quark, Inc.. XTensions are software tools that are loaded within the QuarkXPress "XTensions folder" and generally appear under the "Utilities" menu of QuarkXPress. They enhance or add-value to the user by adding features, functions and utilities that otherwise may never be introduced to the main application.
To be able to be an official XTension developer, one must be approved and sign an agreement with Quark, Inc..