Your plugin may need to get data from the application, such as submissions, issues, authors, users and files. Use the Service classes to retrieve information.
$submissions = Services::get('submissions')->getMany([ 'contextId' => $context->getId(), 'count' => 20, 'assignedTo' => Application::get()->getUser()->getId(), ]);
If a Service class does not exist for the data you want, you may need to use a DAO.
$authorDao = DAORegistry::getDAO('AuthorDAO'); $authors = $authorDao->getBySubmissionId($submissionId);
View more examples.