Persistence 

This folder contains the following file:

Configuration/Extbase/Persistence/Classes.php

Classes.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Extbase/Persistence/Classes.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Extbase/Persistence/Classes.php

EXT:my_extension/Configuration/Extbase/Persistence/Classes.php is used to configure mapping between a database table and its model. The mapping in this file overrides the automatic mapping by naming convention.