The Merge lessons option has replaced in mAuthor the previosuly available Subset of pages functionality. It allows creating a new lesson from a set of pages selected from a different lesson or multiple lessons. What is more, the Merge lessons functionality allows multiple users to work simultaneosuly on one lesson. First they create different parts of the lesson, and then their work is merged in one.
More information on how to use this new functionality is available in Merging lessons section of mAuthor documentation.