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Webflow provides two types of paid plans. One is a site plan (hosting) and the other is a workspace plan. They are different things.
You always need to pay for the hosting (your client in reality), but you don't necessarily need to pay for the workspace plans.
A site plan (hosting) is linked to each website you create in Webflow. It's individual for every website. And workspace plan is for your Webflow account. Where you log in, and have all those websites you've built.
Hosting is simple. When you need to publish a website on a custom domain, you need to pay for the hosting. That's all there is to it in a nutshell.
As for workspace plans, you can pay for it or you can use it for free.
On a free workspace account, you can only have two unhosted websites (projects). But you can have as many paid/hosted sites as you want. If you pay for the workspace then you get more free/unhosted projects and some other features like project transfer. That's the main difference.