I know I can embed but would like to know more about these options and how to use them:
I could offer the link to the reference file from MerlinProject version 7: https://www.projectwizards.net/documentation/merlin-project/reference/en/index.html#_combine_projects
It explains the typical use case (Create sub-project) and reasons why you might want to choose the alternative (Copy Content)
Best regards Vicky