via In-tools.com Blog
This has been a source of a lot of grumbling over the years. I’ve heard all kinds of silly theories as to why this is so — including one that Adobe does this on purpose to “force” you to upgrade. Conspiracy theories are exciting and everything, but there’s really no conspiracies related to InDesign…
This issue will be brought to the forefront once again with the release of InDesign CS5.5. InDesign CS5 will not be able to open InDesign CS5.5 files. Here’s an explanation as to why:
The InDesign file format is in reality one of a database, and InDesign is a (very complex) transactional database application. Every action in InDesign is a database transaction and all these transactions are saved as a database file for InDesign to be able to read. The fact that InDesign works this way allows for many of its advanced features such as unlimited undo and its almost flawless file recovery. (Interestingly enough a lot of “fluff” is saved in the file resulting from previous transactions which get cleared out by doing a “save-as”.)
Continue reading at What’s with back-save to earlier versions of InDesign? | in-tools.com.