A product page for a bathroom sink states a "rough in valve" must be purchased separately for the purpose of installation. I wasn't sure if I really needed that because I am just going to replace my current faucet and there should be an existing valve under the sink, but the check out page reiterates that the valve is required. I did a bit Googling and on another forum some folks also mentioned the necessity of the valve. The product page claims the valve, by the same company, is specially designed.
I am stumped because it seems another faucet model from the same manufacturer does not require a valve. And the views on that other product page make no mention of a valve requirement and talk only about how easy the installation is. So do I need to purchase a valve for a faucet replacement job? Do some faucets require special valves? In what ways if any are the two faucets different in installation?
Also what's the difference between valve assembly and drain assembly? Do they ever overlap? I know what a valve is, but I am not sure what valve assembly and drain assembly are respectively. Google and Google Images have left me more confused as they seem to give various pictures of different things.