TableRegistry::delete

In some cases you want to be able to delete just one instance from the TableRegistry. 

For example, you have two separate databases with user info (one global database and one organization specific database). 

We have App\Model\Table\UsersTable for the global users table and 
Organization\Model\Table\UsersTable for the organization specific users

If we want to check the credentials on the organization database first we use TableRegistry::get('Organization.Users'). After this TableRegistry::get('Users') return the UsersTable from the Organization plugin. If we want to check the global database, we have to use TableRegistry::clear() to make TableRegistry::get('Users') return an instance of App\Model\Table\UsersTable. Now all configured instances are gone from the TableRegistry

TableRegistry::delete() makes it possible to remove just one instance from the TableRegistry.
You don't need to call isset() here. unset() alone will work fine and not emit any errors.