I'm not sure if compatible is the most accurate way to describe this. You will be able to open an IDML file exported from CS6 in CS5, but it won't necessarily be exactly the same as a document created in CS5. Any features that were introduced in the later version that were not present in the older one will be converted to something compatible with the older version's code. I don't have CS6, so I can't site a specific example, but back when I used Quark, they introduced Type on a Path in a version, and if downsaved to the previous version that didn't have type on a path, the object was converted into type within a rectangle, which looks nothing like it did in the newer version.
Manish_1988 wrote:
…compatible with CS4 and later version
Backsaving has historically only worked with the previous version, so I'm not sure that CS4 will open a CS6 IDML or not.
