With the file extension IDML, documents of the DTP (Desktop Publishing) solution InDesign are marked in the so-called InDesign Markup Language Format. With InDesign, for example, you can create print templates and layouts for print products such as flyers, catalogs or brochures.
Imdl files are documents (extension indd), which were saved via the export function of InDesign via 'File -> Export'. An imdl file is a ZIP-compressed archive containing various XML files. So besides the designmap.Xml, which specifies the global document settings and the document layout, there are also Graphics.Xml, Fonts.Xml and Styles.Xml.
The IMDL format is designed for sharing between different InDesign versions and was introduced with the CS4 version as a replacement for the INX format, but can not be processed by earlier versions.