Events ​

A SmartPy contract can emit so-called events. An event can be emitted with the following command:

sp.emit(event, with_type = True, tag = None)

The event argument can be of any (packable) type and represents the event to be logged. The optional tag argument is a string that is logged along with the event. The option with_type determines whether the event's type is repeated in the generated EMIT instruction. Only in some rare situations involving very large types you may want to set it to False.

See the Tezos documentation for further details. Michelson EMIT