The central area of the ACGUI window is dedicated to editing ASN.1 schema definition files. To create a new file, click the "New" button in the toolbar or select "File->New Schema File" in the menus. To open an existing schema file, either click the "Open" button in the toolbar or select "File->Open File..." from the menus. In both cases, the file will be added to the project and the editor will show it. Clicking on an ASN.1 schema file name in the project window will also display that file in a tab in the editor.

At any point during editing, the schema can be saved and checked for proper syntax by clicking the "Validate" button in the toolbar.