Submodules are a common way to include code by referencing another git repository. You can read more about submodules on the official Git SCM site. We support the use of public git submodules, however private git submodules, i.e. submodules which you need to authenticate to access, are not supported.
Because changes to public submodules can sometimes result in unwelcome surprises in the future, we don’t recommend their use when you have little or no control over the submodule. Please review our best practices on using plugins.