Can someone clarify what this means in the release notes? "users

Jorge,
I believe what you are referring to is the *post tab*, which is selected by default when one clicks in the “Click here to post…” at the top of the News Feed. When posting under the post tab only your followers will see your post, however, if you post under the message tab you can direct your post to groups, or even specific users - who will see your post/message regardless if they follow you. In other words, following users is not the only way to see activity on your News Feed – you will see all posts from feeds you are subscribed to as well as any messages directed to groups you belong. Please see forum.appian.com/.../News for more information.

All posts are inherently open and public and discover-able through search but users only see posts of users they are following in their default activity stream.

This is the line that is confusing. It reads as if a user will see posts only after they follow that user. In other words, in order for a user to see my posts, they would need to follow me or search for my posts. 

Also what is the default activity stream?

I agree the statement is confusing. The Appian forum is an open area for all users to post and respond to posts. So i would think the default action would be to "post a question and the default be it be viewed bu everyone. If i want to narrow the exposure of one of my posts i should have to configure tempo to narrow exposure not for it to be the default.

Yes, its a trade off. Imagine in a corporation of 10,000 users if all users saw all content by default (even if public) it would be a lot of noise. In Appian you only see in the default activity stream (i.e. News > All) messages that are sent to you or your groups, or posts that are made from people that you follow. You can still search for posts or public directed messages not to you or your groups, but they will not just pop up in your feed.

Thanks for clearing that up. Yes there are pros and cons for both sides.