The person above me did a great job describing how to do it. The main point of a custom for those who psl is that if you have a CDJ or psl journal and don't want your threads or discussion posts to be seen by just anyone.
For example, if I use a custom for player X and I, then player Y who I don't really know can't go snooping around into what X and I are up to. And for some players having that extra security when meeting new people is important. I'll do it to spare those on my fpage from having to scroll past me when I'm chatting with someone.