EXT

EXT
Path

$GLOBALS['TCA'][$table]['ctrl']

type

array

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)
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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.