UGC in a different tree

Not only ACLs, but also replication (synchronization).

Site content is entered on an author instance and then published using replication.

User generated content (UGC) is entered on a publish instance and then needs to be synchronized.

For AEM 6.0 social communities and earlier,

•  synchronization of UGC among author/publish instances was through replication, and was complicated, especially when the topology was a publish farm.
• moderation of UGC occurred mostly in the author environment, which is typically behind a firewall and not accessible to community members, although AEM 6.0 social communities supports "in-context" moderation by trusted community members.

As of AEM Communities 6.1,

•  UGC is not only in a different tree, it is likely in a different repository.
•  UGC is no longer replicated and instead relies on a common persistence store, such as MongoDB or Adobe Social cloud, while the site content remains in JCR.  
•  UGC is accessed through the Social Resource Provider (SRP) API so that custom code development need not be aware of the underlying deployment.
•  trusted members have access to a site-specific moderation console in the publish environment.  

Another feature where site content and UGC are treated differently is translation.  UGC can be translated separately from the language of the current page, and merged into one stream of UGC, such that each language copy of a multi-lingual site will display comment threads that are the result of merging all entries from the different language copies for that thread, so the conversation is truly global.

Hope that helps.

- JK

BTW - for an overview of AEM Communities 6.1, visit http://docs.adobe.com/docs/en/aem/6-1/administer/communities/overview.html