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<?php 

// this would go in your theme functions.php file

/**
 * Custom product meta field to use as barcode
 *
 * By default WooCommerce POS will use the SKU field- '_sku'
 * You can change the meta key using the following WordPress filter
 * 
 * Below are meta fields from popular barcode plugins
 * '_ywbc_barcode_value' - YITH WooCommerce Barcodes Premium
 */

function my_custom_pos_barcode_meta_key(){
  return '_barcode'; // change for your custom meta key
}

add_filter('woocommerce_pos_barcode_meta_key', 'my_custom_pos_barcode_meta_key' );