What is the $node->iid suppose to be in Acidfree.module?

 function _acidfree_get_thumb_url(&$node, $size='thumbnail') {
    
    switch ($node->type) {
    case 'acidfree':
        return _acidfree_get_thumb_url(_acidfree_get_album_thumb($node));
    case 'image':
        return file_create_url($node->images[$size]);
    case 'video':
        $image = acidfree_get_node_by_id($node->iid);
        return file_create_url($image->images[$size]);
    }
}

function _acidfree_get_thumb_path(&$node, $size='thumbnail') {
    switch ($node->type) {
    case 'acidfree':
        return _acidfree_get_thumb_path(_acidfree_get_album_thumb($node));
    case 'image':
        return file_create_path($node->images[$size]);
    case 'video':
        $image = acidfree_get_node_by_id($node->iid);
        return file_create_path($image->images[$size]);
    }
} 

Is it suppose to be $node->id ?

The reason that I'm asking is that I want to know how acidfree finds the thumbnail from a content type. For example, the Image module generates a thumbnail. However, I'm using FlashVideo and I would like to manually attach an image to the video node. Then when I add the video to an Acidfree album, it should show a thumbnail as a link to the video node.