It's pretty straightforward: Customers who use Yahoo! Web Hosting appropriately can have access to unlimited disk space, data transfer, and email storage, depending on their plan.
What is appropriate usage?
Yahoo! Web Hosting is designed to serve the web hosting needs of small, independently owned and operated businesses in the United States. It is not intended to support the greater web hosting needs of large enterprises or internationally based businesses.
Yahoo! Web Hosting is also a shared web hosting service, which means that a number of customers' web sites are hosted from the same server. To ensure a reliable service available for the greatest number of users, a customer's web site usage cannot adversely affect the performance of other customers' sites.
Additionally, the purpose of Yahoo! Web Hosting is to host web sites, not store data. Using an account primarily as an online storage space for archiving electronic files is prohibited.
So what does "unlimited" mean, really?
With one of the unlimited Yahoo! Web Hosting plans, you can create as large a site as you like (you won't face an upper limit, or "ceiling"), but we will place some constraints on how fast you can grow. In other words, you can add as much content as you want, but maybe not all at once. The vast majority of our customers' sites grow at rates well within our rules and will not be impacted by this constraint.
In most cases, if you use an unlimited Yahoo! Web Hosting service appropriately, visitors to your web site will be able to download and view as much content from your site as they like. However, in certain circumstances, our server processing power, server memory, or anti-abuse controls could limit downloads from your site.
You can also upload as much as content to an unlimited Yahoo! Web Hosting as you like, subject only to the rules that control how fast your site can grow (see above).
Unlimited email storage gives normal email account users like yourself an opportunity to not have to worry about hitting a storage limit. You can save your correspondence and never worry about having to delete older messages to make room for more.
However, the purpose of unlimited storage isn't to provide an online storage warehouse. Usage that suggests this approach gets flagged by Yahoo! Business Email anti-abuse controls.