WordPress at a glance
function is not described

_wp_check_split_default_terms() WP 4.2.0

Check default categories when a term gets split to see if any of them need to be updated.

No Hooks.

Return

Nothing (null).

Usage

_wp_check_split_default_terms( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy );
$term_id(int) (required)
ID of the formerly shared term.
$new_term_id(int) (required)
ID of the new term created for the $term_taxonomy_id.
$term_taxonomy_id(int) (required)
ID for the term_taxonomy row affected by the split.
$taxonomy(string) (required)
Taxonomy for the split term.

Code of wp check split default terms: wp-includes/taxonomy.php WP 5.2.2

<?php
function _wp_check_split_default_terms( $term_id, $new_term_id, $term_taxonomy_id, $taxonomy ) {
	if ( 'category' != $taxonomy ) {
		return;
	}

	foreach ( array( 'default_category', 'default_link_category', 'default_email_category' ) as $option ) {
		if ( $term_id == get_option( $option, -1 ) ) {
			update_option( $option, $new_term_id );
		}
	}
}