We’re getting more technical here so hold on but we can make this simple.
Some things roll up (into the parent community)
So we’ve established that a subdomain is part of the larger ecosystem of the parent community, right? Because of this, when resources are posted within a partner organization’s subdomain, those resources CAN roll up to the larger community should (1) the partner organization wish for that to roll up to the larger community, and (2) the larger community admin approves that item to roll up to the larger community.
Nothing rolls down (from the parent community)
Your subdomain is your environment and while you can choose to push some of your resources up to the larger community, the larger community cannot push resources into your subdomain. When a user visits your subdomain they will only view your organization’s content.
There’s a User Filter
Now, there is a filter on each of your resource pages that will allow a user to see content from the larger community should they change that filter, but the default is set to only show your organization’s content. For example, if someone lands on your content library page they will only see your content, BUT there is a filter to allow that user visibility to all resources in the larger community.
The parent community is one database of information hosted in the cloud. A subdomain allows a filter of a huge amount of data to only display items hosted/posted by your organization. Each subdomain organization has access to the data of their organization, and only their organization.