option "placeholder" added to support custom placeholder elements.

option "placeholder" can be used to support custom placeholder elements.

this is especially useful when table row (TR) elements being sortable. 
without this option you cannot use the plugin for TR dnd.

with this option you can use the plugin like this:

$( 'tbody').sortable({
    placeholder : $('<tr><td></td></tr>')
});

and table row dnd is enabled.