You own IP you create. I'm no lawyer and this isn't legal advice, but I think that is universal and doesn't need to be specified. For example, if you create images with Photoshop, you own those images. For Spine, if you create a Spine project file or export JSON, binary, images, or video or create a texture atlas, you own those assets and can do anything you want with them: sell them, give them away for free, etc.
Where Spine differs from most software is that we also provide the Spine Runtimes when you license the Spine editor. These are libraries which are integrated into your application to load and display Spine JSON and binary data and images. Going back to the Photoshop analogy, you don't generally need anything special to use images made in Photoshop in your app
most apps know how to load and display images. For Spine, loading and displaying Spine JSON and binary data just as it appears in the Spine editor is quite complex, so we provide the Spine Runtimes to do so.
When you license Spine, you can use the Spine Runtimes in any number of apps. For Spine Essential and Spine Professional users, your Spine license lasts for life, so you are good to go. However, if at some point you make more than $500,000 USD in a year, then you need Spine Enterprise which is licensed annually. If you choose not to renew the annual Spine Enterprise license, then you must also cease distributing applications which contain the Spine Runtimes. Some users dislike this annual renewal, so we provide the option to make a one-time payment per product which allows the Spine Runtimes to be used in that product perpetually.
Note if you want to use images, video, or texture atlases in your app, then you don't need the Spine Runtimes, though you also won't have many of the benefits of skeletal animation (small size, animation mixing, dynamic behavior, etc). Also, if you want to write your own libraries to load and display Spine JSON or binary data, you don't need the Spine Runtimes. However, please be careful to create a clean room implementation of your own runtimes. If you are using any pieces from the Spine Runtimes then it's the same as using the entire Spine Runtimes.
We put enormous effort into maintaining and updating the runtimes for many languages and game toolkits, and to support the latest Spine editor features as soon as they are available. For most people, using the Spine Runtimes is the way to go.