1. When I get plugin via plugin manager it set self as service locator (https://github.com/zendframework/zf2/blob/master/library/Zend/ServiceManager/AbstractPluginManager.php#L66).
And plugin has plugin manager as service locator. For example - FormElementManager.
2. Developer can create a tree of plugin managers (sometime i do this, for example i create ModelsManager and ModelsHelperManager for helpers for each model)