I think here you mean, main and sub microflow.
The main microflow allows you to express the logic of your application. It can perform actions such as creating and updating objects, showing pages and making choices.
A sub microflow is something which you can make when building large and complex logics. You can separate a part of of your logic in a sub microflow. It makes it manageable and also reusable.
Here is a document on how to extract sub microflows and use them.