Autoloading Contao 4 or Symfony bundles
Contao 4 is a full-featured Symfony application. Extending a Symfony application is accomplished using so-called bundles, which can extend the configuration at compilation-time. Read more about bundles in the Symfony manual.
Because Contao Managed Edition does not know what bundles exist and in what order they must be loaded, the Manager Plugin needs to tell that to the application.
This information is incomplete.