Custom Barcode Field
By default WCPOS will use the SKU field as the barcode.

WCPOS Pro users can use any product meta field by selecting a Barcode Field in WP Admin > POS > Settings > General

Custom barcode meta fields can also be set by using the woocommerce_pos_barcode_meta_key filter.
functions.php
/**
 * Custom product meta field to use as barcode
 *
 * By default WCPOS 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' );
important
A barcode must be unique for each product.