one of the documented procedures in this installation of the ACS
dp_insert_checkbox   name   values   { on_value "t" }   { hidden_field_name "_cv" }
What it does:
Inserts a checkbox and marks it if necessary (the value is on or Y). Also inserts a hidden field to record an uncheck in the box if necessary. Note that the name _cv stands for _check_vars, but is abbreviated so as to not hit the limit in the size of a get too easily.
Defined in: /web/philip/tcl/data-pipeline-defs.tcl

Source code:

    if { ![empty_string_p $values] } {
	for {set i 0} {$i<[ns_set size $values]} {incr i} {
	    if {[ns_set key $values $i] == $name} {
		set value [philg_quote_double_quotes [ns_set value $values $i]]

    set str "
<input type=hidden name=\"$hidden_field_name\" value=\"$name\">
<input type=checkbox name=\"$name\" value=$on_value"
   if { [info exists value] && $value == "$on_value" } {
       append str  " checked"
    append str ">"
    return $str