Their CDN has two tiers, a super-cheap one (0.005/GB) with only 10 nodes and a more expensive one (0.01/GB) with 100+ nodes. The CDN and the storage services are distinct. The storage service is priced per quantity of data stored and replication zones, the CDN is priced at data served and geo-redundancy. You use FTP to manage the storage, not an API. A CDN can pull from a storage, or from a live website. Each CDN gets a b-cdn.net subdomain and you can either CNAME your own [sub]domain(s) to it or link it strictly for your static assets.
You load money in your account (minimum of $10 per load) and at the end of every month they take how much you’ve consumed (minimum of $1 per month).
In my case I only have a few hundred MB in total so I generate the websites locally and upload the static snapshots to their storage and serve from there with the main website domain CNAME’d to the CDN domain. But they have tutorials for acting as a static cache for WordPress for example or other CMS.
The CDN’s have lots of useful settings like redirect/block rules, you can assign a free SSL cert, can do CORS headers, hotlink protection, custom error pages, control the cache timeouts, concurrent requests, apply all kinds of limits, you can white/blacklist countries, control regional routing and so on and so forth.