Hans Holo is correct.
I'll add some more information though.
Beatinfected says:
if I understand aright it means that an uploaded track has no shadow-information in it and the one who download it has to calculate them again in any case?
Actually, there is always shadow information included in the track file, if it's a validated track ready for people to play.
You can easily look at the filesize to see if the shadow information is there. If it has shadows then it will be at least several hundred KBs in size. If it has been saved without shadows calculated (which is sometimes the case for a work-in-progress track) then the track file may only be around 20 to 50 KBs.
But once the track is done, validated, and saved... there are some things to keep in mind...
If the author calculated "fast" quality, then that is the highest quality shadows/lightmap that is included in the track file.
But if the author calculated "high" quality shadows, it enables higher quality computations. The "default" quality shadows saved with the track are a higher quality as a result. But they're still seen as "default" quality by the game.
The person who downloads the track has an option for what is the lowest shadow quality that they are willing to play with in a track. This setting is in your launcher settings under display -> compatibility. It's called "Lightmap quality". Whatever you set this to will be the lowest acceptable shadow quality you will play a track. If the author has calculated shadows lower than the setting you chose, then the game will calculate shadows (at the time of loading the map) to match whatever setting you picked so that you don't play at a lower setting than you wanted. If the author chose a setting higher than what you've picked in your lightmap settings, then the game will not calculate shadows at the time of loading the track to play it because higher quality shadows are already provided by the author.
So for instance, if you have "Lightmap quality" set to "High" then the game will always calculate shadows on every track you download and play, because the maximum quality saved in any track file is "Default" quality. So it has to always calculate the higher quality shadows.
However, if you have the "Lightmap quality" set to "Very fast" (as I do), then the game will accept any quality shadows that are calculated by any author and saved in the track file. Because, this is telling the game that you're willing to play tracks with "Very fast" shadows or higher.
As I understand it though, if higher quality shadows are provided in the track file, then the game will use those, even if you have set the "Lightmap quality" in your launcher settings to a lower setting.
Keep in mind this is all based on experimentation and conjecture based on many tests. This is not an official report of how it works. This is just how I understand it to work.