Answer: Some boards have private sections that you can only become a member of once you have a certain number of posts on that board. To find out how many posts you have on a certain board, you can use the search function.
At the top right of each forum, below the subforums and above the topics, click on Search this Forum, then select Advanced Search.
In the search forum, you need to enter your user name under Search by User Name, then change the Show Results As in the bottom left to "Posts." Once you have done these two things, click Search Now.
This will give you a list of all of the individual posts you have made on that board. But don't worry - you won't need to count! At the top right, it will tell you how many results there are; this is the total number of posts you have made on the board. It will only pull up to 500 posts, so if you have more than that, it will only give you the most recent 500 posts.