/usr/include/ufs/ffs/fs.h contains the following information,
which seems to answer your question.

/*
 * Grigoriy Orlov <gluk@ptci.ru> has done some extensive work to fine
 * tune the layout preferences for directories within a filesystem.
 * His algorithm can be tuned by adjusting the following parameters
 * which tell the system the average file size and the average number
 * of files per directory. These defaults are well selected for typical
 * filesystems, but may need to be tuned for odd cases like filesystems
 * being used for sqiud caches or news spools.
 */
#define	AVFILESIZ	16384	/* expected average file size */
#define	AFPDIR		64	/* expected number of files per directory */


On Sun, Feb 16, 2003 at 09:03:08AM +0100, Wojciech Puchar wrote:
>      -h avgfpdir
>                  The expected average number of files per directory on the
>                  file system.
> 
> 
> what's an exact effect of that option?
>