Example of how to force promotions to only apply to a single event within the cart

tw_ee_promotions_only_single_applicable_item.php

<?php //Please do not include the opening PHP tag if you alreayd have one.

add_filter('FHEE__EED_Promotions__get_applicable_items__applicable_items', 'tw_ee_promotions_only_single_applicable_item', 10, 5);
function tw_ee_promotions_only_single_applicable_item(
    $applicable_items,
    $promotion,
    $redeemable_scope_promos,
    $events,
    $cart
) {
    if(stripos($promotion->code(), 'CART') !== false) {
        $first_applicable_item[] = reset($applicable_items);
        return $first_applicable_item;
    }
    return $applicable_items;
}