Activity Description in FIT File?

I´ve seen Platforms that somehow got the Description of an Activity from Garmin Connect.

I´m wondering how they do this, as I´m under the impression, that the FIT Files are not containing this information (as it´s later added on the Garmin Connect Website, when the File is already created). But I found no clue that this description is available via the API aswell?

Cheers iTim