Disks have a size. Networks have a capacity. Nobody sells you infinity, because infinity does not fit in a rack.
What "unlimited" means in practice is "we have not written down a number, and we reserve the right to decide later".
The Clause Doing The Work
Every unlimited plan has an acceptable use policy attached. It will say something like: resources must be used for the normal operation of a website, and accounts consuming disproportionate resources may be suspended or migrated.
Disproportionate is not defined. That is not an oversight, it is the point. A defined limit is a promise. An undefined one is discretion.
Why It Usually Does Not Bite
To be fair to the model, most customers never hit it. The average small business site is a few hundred megabytes and a modest amount of traffic. Unlimited works because the overwhelming majority of people use very little, which subsidises the few who use a lot.
The problem is not the average case. It is that you find out where the line is at the worst possible moment. When your site has just been mentioned somewhere and traffic has spiked, or when your backups have quietly grown for two years.
What We Do Instead
We publish the number. Ten gigabytes on Starter, thirty on Business, and so on up the range. It is a smaller-sounding figure than "unlimited", and we lose some comparisons because of it.
But a number you can check against your actual site is worth more than a word that means whatever we decide it means. If you are near your limit we will tell you, and you can move up a plan or clear out old backups. No suspension email that arrives as a surprise.
If a host will not tell you the number, they have one. They just do not want you to compare it.
