list of subscriptions, description "Unknown"

I too have the
Unknown Node type : Unknown Not available

FWIW, in my case it seems to be a problem with CCK content types and organic groups.

Drupal 6.13,
Content 6.x-2.5,
Notifications 6.x-2.1,
OG 6.x-1.3,

Okay, a little bit of digging ... it maybe looks like the problem might be that the function og_notifications_notifications() doesn't implement the case 'subscription fields'.

The 'Unknown' part I fixed by adding (1) this function to og_notifications.module:

function og_notifications_group_description($nid) {
$group = node_load($nid);
return $group->og_description;
}

and (2) adding the following case to og_notifications_notifications():

case 'subscription fields':
// Information about available fields for subscriptions
$fields['group'] = array(
'name' => t('Group'),
'field' => 'nid',
'type' => 'int',
'format callback' => 'og_notifications_group_description',
// 'value callback' => '', // what to put here?
);
return $fields;

The problem with 'Not available' is because I have notifications enabled only for nodes that are posted to a group, so that nodes of type 'nodetype' are not enabled, but all the nodes are looked up with type 'node'type'. Don't know how to fix that yet.

the "unknown" part of this bug seems to be fixed in 2.1. the "Not available" part is a duplicate of this report which is more clear.