Ah that must be new, used to be only parcels did.
They used to generate a number (same as letters do) but when you tried to track it it said tracking was not available for that service.
Just checked with a 2nd class large letter I posted a few days ago - granted not through click and drop but through Intersoft which is integrated into our RM OBA and I get the same thing : “Tracking information is not available for this service”
Even their website states a delivery confirmation is only for parcels sent using 1st/2nd class.
Maybe a new thing they are trialing