New posts gets all the posts marked as new. How posts are marked as new depends on your options settings. With the minimum setting, new is based on your last visit time, and a cookie that tracks posts viewed this session, so new posts gets all posts since your last activity of your last visit that you have not viewed this visit. At the maximum setting, a record of your post viewing is stored in the database for months, and new posts gets all posts since the cuttoff date that you have not viewed in any session. In this case, there is a separator that shows which posts are new since your last visit.
vBulletin options|General Settings|Thread/Forum Read Marking Type
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