I believe the letters you're talking about are referring to the era! So, in the business directory they show which eras the business is available to play in, and in character profiles, people can do the same to show what timelines they play the character. F is Founders, V is Victorian, E is Expedition, and so on.
You can use this feature by clicking the little button on the left of the post title when writing a new topic.
Hope this helps! :)