IDML also refers to the IDML-based .IDML (IDML export, or InDesign Markup) document format, which uses a ZIP container. IDML export format is intended to be more portable than the native INDD format. It is the successor to INX.
IDML might also refer to the whole family of IDML-based formats, which includes .IDMS (Snippets), .ICMA (InCopy assignment), and others.
.IDML files normally have ASCII "
mimetypeapplication/vnd.adobe.indesign-idml-package" at offset 30.