The first link is related to the In-Process Alfresco SDK that it allows you to extend the platform in order to deploy custom models, components and REST APIs into the same Alfresco Repository process. This is the core tier of the platform and I would suggest to look at this initially to understand if you need to add some of these extensions to your project.
The second one is dedicated to extend the ADF platform for adding custom Angular components to the Alfresco Content Application or to the Alfresco Digital Workspace for the Enterprise edition.
I would also recommend to look at the Out-Of-Process SDK for managing event programming outside of the Alfresco Repository instance using a standard Spring Boot application:
https://docs.alfresco.com/content-services/latest/develop/oop-sdk/
Hope this helps 
