Every newer version can read files from an older version of InDesign. An older version is not able to read native files from the newer version, although IDML files can be opened from Id CS4 and newer. But for workflows where files should run between computers it is not recommended.
And please read this informative article on why New Files from new version of InDesign cannot be read in older versions of the software.
IDML is not there as an all-purpose "version save" - it's for clearing out problems from files converted from Quark or Pagemaker etc.
See the help files
Save an InDesign Markup (IDML) document
In some cases you may want to save an InDesign in interchange markup format. It’s especially useful to save in this format when you open a QuarkXPress or PageMaker publication in InDesign, when you open a document created in a previous version of InDesign, or when you’re experiencing problems with your document, such as not being able to delete color swatches.
- Choose File > Save (CS6) or File > Export (CS5).
- From the Save As Type (Windows) or Format (Mac OS) menu, choose InDesign Markup (IDML).
- Click Save.
You can open the saved IDML file in InDesign CS5 and InDesign CS4 but not in any previous version of InDesign.
For information on using IDML for development purposes, see www.adobe.com/go/learn_id_indesignmarkup_cs4_en.
Thanks guys. Still have my suspision though on CC to CS6 compatability when dealing with InDesign files. I've historically back-saved through all the versions of the Creative Suite.
Think a trial download of InDesign CC may be the best call in this instance.
Waht suspicions? CC is a new version, and just as inthe past you will need to export .idml to open in earlier versions.
Your only "upgrade" path is to Creative Cloud which includes InDesign CS6 and InDesign CC. For work with those still on CS6 you'll have that version available to you so you won't have to worry about IDML backsaves.
Thanks Bob, that was the clarification I was looking for.