You can find it being set in bbs.scm:update-post-count
(set-cdr! old-date (string-append date " *"))then find the update-post-count invocation in post-message
(body (http-request-body req))
(params (parameters->alist body))
(vip (assq 'vip params))
[...]
(if vip
(update-post-count board thread date post-number)
(update-thread-list board (string->number thread) date post-number))