Whole House Tankless Water Heater
Whole house units mean that they have higher GPM flow rate capacity and can handle demand for more than one fixture at a time. For example a unit may handle two shower fixtures at one time or a dishwasher, kitchen sink and lavatory hot water faucet at one time. Why the different number of fixtures? Because different fixtures use different amounts of water. Some shower heads can use six times more water than a bathroom lavatory faucet.
So the size and number of whole house tankless water heaters you need will be largely driven by flow rate and that is determined by the number and types of fixtures you may have running at one time. And the worst culprits are shower heads. That's why you may need more than one whole house tankless water heater hooked up in parallel to meet your hot water demands, especially for simultaneous shower usage.
Whole house units are much more expensive than point of use units and cost from several hundred to several thousand dollars (without installation).