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.