one of the documented procedures in this installation of the ACS
apply_content_mask   tag
What it does:
Returns the bitwise AND of the content tag arg with the currently connected user's content preference
Defined in: /web/philip/tcl/ad-content-tagging.tcl

Source code:

    set user_id [ad_get_user_id]
    if { $user_id == 0 } {
	# not logged in 
	set content_mask [ad_parameter DefaultContentMask "content-tagging" 0]
    } else {
	# logged in
	set content_mask [util_memoize "naughty_content_mask_for_user $user_id" [ad_parameter UserContentMaskCacheTimeout "content-tagging" 600]]
    return [expr $tag&$content_mask]