Marketplace is a place, where you can download a whole application, or merge schemas into an existing one.

When you create a new application, there is nothing to explain. Just follow the instructions.

However, what to do, if you have already an application and you just want to merge schemas from an existing one into it?

As an admin, you can add a new schema by the blue button in the bottom part of the list of presented schemas.

When you go to Marketplace and select download for one of the applications, its schemas will be appended into the open application.

The same way you use for updating schemas.

So, when you want to update schemas to the latest version, just open the application. Then choose Add new schema and download it again from Marketplace.