German Police & Customs on ALE

The German Police and Customs have a very active ALE net on HF and it's a great way of logging some of the rarer and more unusual ship and coast stations. This is the first transmission I logged, a call from German Police Boat Bad Dueben BP23 to Police HQ:

 

and here is the vessel herself:

The two frequencies I have heard active are 8132khz and 4553.5khz. I have yet to hear any voice comms, only ALE transmissions. Perhaps someone can enlighten me: the ALE equipment, having determined the optimum frequency from a longish history of ALE soundings, uses this information for what purpose? If it's to determine what frequency to use for ongoing comms, where and in what mode is this happening? I'm not hearing any actual information exchange/traffic between stations, such as with the Italian Navy who will use voice mode to establish comms before exchanging STANAG messages.

Anyway, I am a newcomer to the data-modes and understand very little. I just know I enjoy it and it's great to have the chance to log ships and coast stations that are not active in voice mode.