I'm nor sure if many people use this feature, but it seems to be pretty useful. Here's the answer from the FAQ page:
» What are my Friends and Foes lists?
You can use these lists to organise other members of the board. Members added to your friends list will be listed within your User Control Panel for quick access to see their online status and to send them private messages. Subject to template support, posts from these users may also be highlighted. If you add a user to your foes list, any posts they make will be hidden by default.
If anybody asks you the same question, you can refer to the FAQ.
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