Integrations that solely use the custom code embed

What do you mean by “However, integrations that solely use the custom code embed functionality will not be included in the Marketplace at this time.”
What can be developed?
Moreover, do you have a toturial for developing an app for the platform? It is not clear to me, app can be developed and integrateed with the platform.

There is a quickstart guide on that site Getting Started (, that phrase you highlighted means that your app in order to be listed must interact with Webflow APIs, like adding a webhook, modifying a cms item and so on