wp-admin/includes/class-walker-category-checklist.php:37Starts the list before the elements are added.
$outputstring$depthintoptional0$argsarrayoptionalarray() public function start_lvl( &$output, $depth = 0, $args = array() ) { $indent = str_repeat( "\t", $depth ); $output .= "$indent<ul class='children'>\n"; }Introduced in 2.5.1. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-admin/includes/class-walker-category-checklist.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.