The different types of meters are defined in the Configuration tab. If you have permissions, go to the Configuration tab and select Meter Types from the hamburger:
Once you're in that section - you can add new meter types by entering in the name at the top of the list and pressing ENTER or clicking Add:
Once they're in that list, they'll appear in the Meter Type dropdown when you're entering in a new meter reading.
Next Topic: Meter Triggers