SSD essentially means: Super Secret Directory.

NOTE:
All of the following rules below apply only if the torrent being added to LzTr cannot be found elsewhere at another tracker (What.cd, Waffles, RuTracker, et cetera, et cetera). Therefore, if you download a torrent from another tracker, DO NOT upload it here and add the SSD tag.

This material should NOT be traded!
SSD stands for Super Secret Directory, or something that is specifically rare or difficult to find. It is a phrase that was used in the old days of LzTr, and has stuck ever since. These albums are generally tagged as ssd.ost. While it is impossible to control the internet and leakage of rare items on public sites, it is in your best interest to not trade SSD material on other sites. Doing so will result in demotion or disabling of your account.

What defines a rare score?
Generally, the main idea of SSD material is 'stuff that you can't commercially purchase'.

Isolated Scores, Bootlegs, and OOP albums
Rips from isolated scores from DVD's are considered rare, in addition to bootlegs. Bootlegs can be from casette, VHS rips, and/or self-made mixes that expand or isolate the score in some way. None of these can be commercially available. OOP albums should be considered well out of print, and should not be able to be purchased new by any major distributor.

Limited Releases (FSM, Intrade, etc.) are considered SSD
Anything that is released in limited quantities (FSM, Varèse, Intrada, etc.) are considered SSD.

Academy Promotional Scores and Recording Sessions
Any Academy Promo scores or Recording sessions are considered SSD. This includes any material given to you directly by composers or any media company.

Expanded and Complete Scores
Expanded and Complete Scores are considered SSD, mainly because the releases aren't commercially available. This includes fake complete scores, in addition to fake bootlegs.