Because of the way InDesign documents are versioned, it's not possible to open them directly in earlier versions of InDesign, InCopy or InDesign Server. For example, a document created in InDesign CC 2015 cannot be directly opened in CC 2014 or CS6.
There are two techniques that can be used to share files where the document creator has a higher version of InDesign:
1) Customers with CS6 (specifically 8.1.0) or above, with access to Creative Cloud services, can use the Simple Saveback service which will allow them to open documents created by someone using a higher version of InDesign CC. There is some further information on this feature here:
2) The creator can save the document to the IDML format and distribute that to the user with the previous version. More information on this can be found here:
Note: content created using features specific to a particular version
of InDesign are sometimes modified or omitted if the document is opened in a
previous version. Carefully review any document opened in a previous version.