one of the documented procedures in this installation of the ACS
ad_partner_var_or_default   var
What it does:
Returns the specified variable for the current parter, unless it's the empty string in which case it returns the variable for the cobrandsitedefault
Defined in: /web/philip/tcl/ad-partner-defs.tcl

Source code:

    set value [ad_partner_var $var]
    if { ![empty_string_p $value] } {
	return $value
    return [ad_partner_var $var "" 0 [ad_parameter CookieDefault partner]]