From Online Manual
If enabled by an administrator, users can choose to ignore any boards on the forum. When a board is ignored, it is still shown on the Board Index, but the New Posts icon will not show up for this board. In addition, new posts will not be shown for an ignored board when using the link Show unread posts since last visit. Neither will they be shown when using the link Unread posts next to the category names in the Board Index. However, when using the link Show new replies to your posts, new posts in an ignored board will still be shown. Also, when entering an ignored board via the Board Index, topics that have new posts will still be indicated with an icon.