Difference between submodules and subpackages?

[dstansby]

I'm trying to use autoapi to document a package, and am getting the API docs grouped in both "submodules" and "subpackages":

Does anyone know

1. The difference between a submodule and a subpackage?
2. How I can group these together (ie. merge the lists above) so that all the second level API docs are in a single list, instead of spread across submodules and subpackages