Whether the model uses a thermoblock, boiler, dual boiler, or heat exchange system.
Thermoblock heats up only a very small quantity of water on demand as it flows through the system. The capacity of the water can be as little as 10mL; the larger ones heat up to 50mL at a time.
Models with boilers heat a larger amount of water and keep it heated fairly consistent. For example, the Rancilio has a 300mL capacity, which is heated all at once and then maintained at that heat. A dual boiler includes two boilers; one for making coffee and one for steaming, which saves time as you don't have to switch modes.
Heat exchanger models are used a lot in commercial scenarios. They use a single boiler but are designed to produce coffee and steam at the same time. The boiler produces steam, while a pipe running water through the boiler is heated on the fly to the right temperature.