(variable, depends on extension)
User-defined content for extensions. You can use this as you like.
Let's say that you have an extension with the key "myext", then it is ok to define properties for:
['ctrl']['EXT']['myext'] = ... (whatever you define)
Note this is just a convention. You can use some other syntax but with the risk that it conflicts with some other extension or future changes in the TYPO3 Core.