What is the difference between Across Framework and Across Platform?

Across Framework is an actual Java library providing all the infrastructure necessary to build Across modules and to run applications based on them.

Across Platform is a set of curated dependencies and Across modules that play well together and help developers assemble applications. It is provided as a bill-of-material POM, extending the Spring IO platform BOM.

Across applications usually use the Across Platform BOM as a basis, to ensure that both third-party libraries as well as common Across modules are compatible.