Files with the .IDML extension contain project data created by the Adobe InDesign software, versions CS4 and newer, saved in using an XML file hierarchy and catalogs packed in a ZIP format. The main catalog contains a file called designmap.xml, which defines the properties and layout of the entire document, while the individual sub-catalogs define document elements, such as colors, fonts, and paragraph styles in the Resources catalog, the document spread view in the Spread or MasterSpread catalog, the text contents in the Stories catalog, the elements and settings such as tags and mapping in the XML catalog, and files in the META-INF catalog, which also includes UCF packs.

The InDesign software, to which IDML files are unique, is made for professional webpage layout design. The IMDL format is a replacement for the older, more complicated INX format – it is also more universal, and is supported by other programs.