Source

File: wp-includes/rest-api.php

function rest_parse_embed_param( $embed ) {
	if ( ! $embed || 'true' === $embed || '1' === $embed ) {
		return true;
	}

	$rels = wp_parse_list( $embed );

	if ( ! $rels ) {
		return true;
	}

	return $rels;
}