wcfad_validate_bxgy_rule_taxonomy.php

<?php
/**
 * Filter BXGY rule by brand taxonomy instead of category
 */ 
function demo_bxgy_taxonomy( $taxonomy, $rule, $rule_id ) {
	$taxonomy = 'product_brand'; // Change this to the taxonomy or tag of your choice
	return $taxonomy;
}
add_filter( 'wcfad_validate_bxgy_rule_taxonomy', 'demo_bxgy_taxonomy', 10, 3 );

function demo_bxgy_buy_categories( $buy_categories, $rule, $rule_id ) {
	$buy_categories = array( 34 ); // This is the taxonomy term ID
	return $buy_categories;
}
add_filter( 'wcfad_validate_bxgy_rule_buy_categories', 'demo_bxgy_buy_categories', 10, 3 );

function demo_bxgy_get_categories( $get_categories, $rule, $rule_id ) {
	$get_categories = array( 33 ); // This is the taxonomy term ID
	return $get_categories;
}
add_filter( 'wcfad_validate_bxgy_rule_get_categories', 'demo_bxgy_get_categories', 10, 3 );