Scope:(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.