The pre-fix we use before our number is our trooper type. The TD means, TROOPER DESERT or as we say SANDTROOPER.  TK is a Stormtrooper pre-fix. I have been luck enough to pick my own TD id number.  The number I use is 2479. They are the first three numbers from my Army ID.  Other troopers of the 501st have also been lucky enough to choose their number. This number is our trooper ID just as it is with all military ID's or any other ID's people use at work to identify them self.

I think one of the most famous of these ID's has to come from "TK-421" from STAR WARS, A New Hope. The scene where Luke and Han ambush two Stormtroopers on the Death Star whilst they stand guard outside the Millennium Falcon.

"TK-421 why aren't you at your post?"

"TK-421 do you copy?"

THE SANDTROOPERS OF MOS EISLEY

Sandtroopers are largely identical to standard stormtroopers except they have additional training in specialized areas. Their armour consists of an 18-piece anti-blaster cocoon shell with heat-reflective coating worn over a temperature-control body glove. The sandtrooper helmet features automatic polarized lenses, a built-in COM link and breathing filters.

Though a desert is, by technical definition, any area devoid of significant moisture, these stormtroopers are specifically designed for high temperature environments. As such, these soldiers have also been dispatched to hot, wet, tropical terrain operations since their cooling units and sealed water systems are easily modified to cope with such areas. Instead of depending on machinery that may not function in hostile environments, sandtroopers make use of local creatures for transportation.

The specific detachment dispatched to Tatooine was the Desert Sands unit, led by senior officer DSS-0956. These elite soldiers had training that emphasized independent thinking. All Desert Sands stormtroopers carried heavy blaster rifles, long-range COM links, and food and water packs.

Sandtroopers wear coloured pauldrons as a designation of rank.

When I troop with my armour I like to wear an orange one, but I have all three.

From the picture of the CHECK POINT in Star Wars ANH you can see all the ranks of the sandtrooper and there coloured pauldrons. A sandtrooper sergeant has a white pauldron while an enlisted sandtroopers pauldron is all black Squad leaders, who lead units of seven troopers, wear orange shoulder pauldrons.

There was a rumour that there was a grey one worn by a dewback rider in ANH. This was a mistake he actually wore a dirty white one.