It's a configuration variable. It's an array that contains every object type and groups of them found in BEdita.
It's created runtime, cached in tmp/cache/cake_be_config file and written to configuration.

It can be read with Configure::read('objectTypes') call. In the view it's also reachable through $conf->objectTypes

Array structure:

array(
...
    '22' => array(
        'name' => 'document',
        'model' => 'Document',
        'module_name' => 'documents'
    ),
    
    'document' => array(
        'name' => 'document',
        'model' => 'Document',
        'module_name' => 'documents'
    )
...
)

where the key 22 is the object_type_id for document object.