In Enhance a "website" is like a "account" on cpanel. A "website" gives the user all the resources limited by their package. If you allow "unlimited websites" you would want to consider the impact on your hosting infrastructure. For example, if you made a package with "unlimited websites" then one customer could buy that one package and then use 100% of your server resources.
Addon domains is a way to allow customers to add multiple (web)sites or apps or domains to their "website" while still being limited for things like ram/cpu/storage per your package limits.
We limit "website" on our packages to 1, while allowing unlimited addon domains, so if they buy a 2cpu/2gb ram/20gb storage hosting plan, they can use all those resources as they'd like with as many sites or apps as they'd like - but only within that "website" account, as addon domains...
A lot of people do it the other way, don't allow any addon domains and allow 5-10-20 "websites" per package. The thing with doing it that way is you need to account for the resource consumption of each "website". This is like the wpengine style of pricing. It's not economically viable to sell hosting at low cost using this strategy, as each "website" is costing you 15 cents per month + costing you the resource limits per "website" X however many "website" the customer rolls out. This would be a suitable setup strategy for higher end hosting - you just need to keep the resource consumption in consideration - buy bigass dedis to reduce scaling costs.
Here's a scenario of using this setup poorly. Say you have a server with 8cpu/16gb ram and you hope to be able to host around 10 customers on there. Now let's say you sell a hosting package that includes 10 "websites" with 1CPU/2GB ram. The first customer you get installs all 10 websites of theirs, which are each now capable of consuming 10 CPU and 20GB ram - more CPU and more RAM than your entire server has available... Now you get a 2nd customer and that customer does the same, now they have 20 "websites" installed on that server and they're able to consume up to 20 CPU and 40GB ram, much much more than the server even has available... How long do you think until a bottleneck is reached in this scenario? At scale it will happen quickly, and the server will have terrible performance before you ever break even for the cost of running the server.
In that same scenario if you had limited each package to 1 "website" with "unlimited addon domains", those first two customers could have their 20 "domains/apps/sites" running on their 2 "websites" and combined could only use a maximum of up to 2CPU/4GB ram. Because they're each getting limited by the package they bought for cpu/ram. The addon domains share the resource limits of the "website" they reside in.
