Electrum protocol supports notifications. when you connect to an Electrum node you have an open socket and a bunch of subscriptions to certain addresses which the node will inform you about when there is any new transaction that involves those addresses.
more info can be found on the documentation:
https://electrumx.readthedocs.io/en/latest/what is Electrum protocal ? is it bitcoin feature or configuration option in bitcoin.conf ?
all the additional functionalities and commands that a certain type of bitcoin full nodes support create "Electrum protocol". all these features are added on top of bitcoin core to make it easier for SPV clients to connect to these special nodes and receive their transaction history, etc. faster and easier.
"bitcoin.conf" file is basic configuration that affect the way you run bitcoin core application and is unrelated to this.