Well share-config-custom contains a lot of configuration related to share forms, document library, repository connection, search, and many more. These can't be generalized in few lines. So you must go through the documentation and read about them.
At a high level, the first configuration section you highlighted, is about the share document library as you can also see the "condition" attribute of the config.
This particular config is used to define aspects visibility, addability, and removabliy. Also define content indicators, actions etc.
All these are itself big topics to explore.
The second part you highlighted, is about the share form configuration, used to configure forms for the cm:content (ooth type) or custom content types that you would create. These forms can be used to edit node properties. Share forms also a large topic to explore.
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When you newly start with alfresco and share, the most basic extension we start is the content modeling and share form configurations to work with content model types, aspects and properties.
You should first go through these docs/tutorials:
Content Modeling and Share configuration
Share config extension point in detail
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You can check this posts as well(old but still useful to understand the concepts):
https://hub.alfresco.com/t5/alfresco-content-services-hub/forms/ba-p/289477
https://hub.alfresco.com/t5/alfresco-content-services-hub/forms-examples/ba-p/290049?attachment-id=3...
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Once you understand above extension points, you can also explore this doc:
https://docs.alfresco.com/content-services/5.2/tutorial/share/doclib/
~Abhinav
(ACSCE, AWS SAA, Azure Admin)
