The activity feed is the definitive place to find out about new comments, issues and feedback requests posted by your teammates, or new replies to existing discussions. When you’re not on the activity feed, you can always tell whether or not there’s anything new by looking for a badge on the Activity Feed icon. A blue badge with a white dot means that there are new discussions or replies, and a blue badge with a number inside means that there are new discussions or replies where you’ve been mentioned (with the number indicating the number of mentions).
Note that CodeStream’s entry in your IDE’s status bar also let’s you know when there are new messages in the feed. A dot to the right of your username means there are new messages, and a number means there are new mentions.
As you might expect, new stuff gets added to the top of the feed, even if it’s a new reply to a very old discussion. CodeStream makes it even easier to identify what’s new by adding a blue border on the left side. For example, here’s a new comment at the top of the feed.
And here is a new reply to a comment. The fact that only the replies have the blue border means that you’ve previously seen the comment itself.
Also note the “See 1 earlier reply”. That means that there's an earlier reply that you’ve already seen. Only new, unread replies are displayed in the main activity feed view, but you can click on that link to see the full discussion thread and add a new reply of your own.
The activity feed goes all the way back to the beginning of time for your team, but if you’re looking for specific older codemarks you might be better off going to Filter & Search.