TIDXPREFIX defines the prefix for the filenames of a multi-page thread index. The filename of the first page of the thread index is always equal to the value of the TIDXFNAME resource. All other pages have a filename starting with the TIDXPREFIX value with the page number and ".html" following. For example, the filenames for a 4 page thread index would be the following:
threads.html thrd2.html thrd3.html thrd4.html
assuming the default values for the TIDXFNAME and TIDXPREFIX resources.