pid and pids search parameters

How do you use the search parameters pids?

from /includes/common/search_proudcts.php I find

$search_data[‘search_product_id’] = empty($pid) ? ‘’ : $pid;

$search_data[‘search_product_ids’] = empty($pids) ? ‘’ : $pids;

I see that pid works like where pid is the product id

index.php?target=products&mode=search&avail=Y&pid=13276 (this works fine)

But how do you use pids (which I assume is a list of product ids)

index.php?target=products&mode=search&type=extended&pids=[what is the format here]

I have tried pids=13276,18711 but it returns an error

Thanks for your help

Is there anyone out here?