What do IDML file types, InDesign and preflight have in common? Markzware, the publisher of FlightCheck software for preflighting, takes a look:

IDML (InDesign Markup Language) is an XML-based format, introduced in Adobe InDesign CS4, for representing InDesign content. IDML is used in several InDesign and InCopy file types:

• IDML files (or packages) are ZIP archives, containing numerous XML files. The entire set of these files represent a complete InDesign document. The files in an archive are described thoroughly in InDesign CS4 IDML File Format Specification. IDML files are identified by the *.idml file extension.
• Snippets are IDML files that describe a subset of an InDesign document. They are not archives, but single XML files that contain IDML. The application UI, scripting facility, and plug-in API allow you to export snippets from InDesign. Most commonly, such a snippet represents a page item or group of page items. These type of snippets usually are identified by *.inds.
• ICML (InCopy Markup Language) files are a special type of snippet. An ICML file (*.icml) represents a single story using IDML.
• InCopy assignment files are another type of IDML snippet. They are used to allow multiple writers to work on one InDesign document. Assignment files use the *.icma extension.
• There are other features that leverage IDML; for example, exporting preflight profiles produces an IDML-based file.
  SOURCE: IDML File Types

There you go, IDML is XML-based, similar to file sets in Markzware’s Ground Controls for the stand-alone preflighting. FlightCheck is the preflight solution that can handle many file types, including preflight for an InDesign file and other Adobe file formats.