TCA Reference

For group, select and inline types (all TCA types that support MM relations)

Description:
MM_oppositeUsage defines an array with maps table names to field names, pointing to possible opposite relations. Since the current relation allows any table, these pointers are checked on creating new MM relations and will apply the MM_match_fields from the accordant opposite configuration. TYPO3 CMS uses this for e.g. sys_category and tt_content relations.

Example:

...
'allowed' => '*',
'MM' => 'tx_myextension_first_second_mm',
'MM_oppositeUsage' => array(
'tt_content' => array('somefield'),
'tx_myextension_domain_model' => array('some_property'),
),
...