one of the documented procedures in this installation of the ACS
ec_user_class_select_widget   db   { default "" }   { multiple "t" }
What it does:
Returns a HTML multiple select list for user_class_id with an option for each user class. Each id in the list default is selected.
Defined in: /web/philip/tcl/ecommerce-widgets.tcl

Source code:

    set to_return "<select [ec_decode $multiple "t" "multiple" ""] name=user_class_id>
    <option value=\"\">none selected

    set selection [ns_db select $db "select user_class_id, user_class_name from ec_user_classes order by user_class_id"]
    while { [ns_db getrow $db $selection] } {
	append to_return "<option value=\"$user_class_id\""
	if { [lsearch -exact $default $user_class_id] != -1 } {
	    append to_return " selected"
	append to_return ">$user_class_name\n"
    append to_return "</select>\n"
    return $to_return