Hi, if your NIS has open harvesting slots and open ports, than any user can request for it to harvest and it will start harvesting for them. This is good for people with small accounts that aren’t running their own node and it is very light extra effort on your node’s part. They will get all the fees and you won’t get anything other than the satisfaction of knowing you helped a small timer.
For your own set up, you can also get the delegated private key from your other accounts via NanoWallet in the harvesting section. Then when setting up an NIS, you can go into the config-user.properties and add those keys, so that the next time NIS starts, you will be harvesting on all your accounts automatically. Please see https://blog.nem.io/nis-auto-start-and-auto-harvest/