That would be confusing to us, as "hot water heater" could mean so many things ! We have instantaneous heaters, of which the gas (or oil) fired combi is one; vented cylinders (the traditional approach); unvented cylinders (needs all the safety stuff to be safe(ish)); thermal stores and heat banks; ...
And of course, each of the cylinders (or storage tanks) can be heated by different means, so that makes lots of permutations.
And head into the plumbing section of some forums, and you'll find supposedly professional plumbers state with pride how they'll never install (or recommend to a customer asking their advice) anything but a combi