Boy, I don't know that I've ever gotten an email in my life with a subject starting with "angle-bracket X close-bracket". I see "[X]" on forum posts and a very occasional email, but maybe the angle-bracket thing is something unique to your work culture?
Sounds like a suggested message tagging mechanism within your company. This way you can easily write a filter to automatically sort, file or highlight he email based on type. My company has a similar thing (ACTION - , QUESTION - , INFO - , etc) but noone ever uses it.
Maybe. I know one person who uses it and then started seeing more. I'm trying to start. I like it as it lets you get an idea for what is going on. Like an extension of shoving the idea in the subject line.