This Primer is unfinished, these are notes for the final product.
Three uses for tags:
security
organization (and therefore tagged items can be – categorized, searched, browsed, or filtered)
site placement (both at the structural level when a tag is a Hub menu and at the data level when an item is tagged, and consequently appears in an area of a Hub)
There is not a notion of being in an interest group. In fact, the existing feature just makes an 80/20 guess that if you start a conversation while viewing a group then you intend that group as the audience, therefore, the group tag is applied by default to the new conversation.