Focus
There are often a lot of requests and responses from a lot of different hosts cluttering up a Charles session. Using the Focus feature, you can separate the hosts that you care about from the ones that you don't.
To activate Focus for a host, right-click on the hostname in the Structure tree and choose the Focus item from the menu. The Structure tree will now show your newly focused host at the top, and then a folder named Other Hosts, which you can expand to see everything else.
On the Sequence view, you can tick the Focused checkbox to only show focused hosts in the sequence.
To see all of your focused hosts, and to edit them, go to the Focused Hosts… item in the View menu.