def callback(track_transfer_batch, user_param1, ...)

Emitted when the batch is started. This will be after all previous batches have finished, which is not necessarily when rb_track_transfer_manager_start_batch is called.

track_transfer_batch :

instance of RB.TrackTransferBatch that is emitting the signal

user_param1 :

first user parameter (if any) specified with the connect() method

... :

additional user parameters (if any)