In this context it refers to Turbo. Often, people will append T to indicate they have the turbocharged version of the engine:

300DT: 300D (sedan) with turbocharger
300TD: 300D station wagon
300TDT: 300D station wagon with turbocharger
...etc. You get the idea.

The car isn't badged this way, but people like to indicate what engine they have, since the 3-liter engine came in both flavors, with turbo and without.

- Nathan
'83 240D, 255k miles
'00 New Beetle TDI, 28k miles