USINGLASTPG is a boolean resource to tell MHonArc if the $LASTPG$ resource variable is in use.
The usage of this resource is only applicable if MULTIPG is set. By default, MHonArc will regenerate all pages of a multi-page index if a new page gets added to the archive. This is done to insure that any links to the last page of the index (via $PG(LAST)$) gets updated.
If you want to save processing time, and you know you are not using $LASTPG$ in any resource (or have a helper CGI program to link to the last page), set USINGLASTPG to 0. Any new page added will only cause the new page and previous page to be written/updated, assuming that added messages always appear at the end of message listings. MHonArc will still update relevant index pages if message listing order dictates such an action.