I believe there should be three separate tags.
- emigration for questions about people leaving their home country
- immigration for questions about people entering a new country
- migration for questions about the routes taken by individuals and families because of historical events (the pull & push factors, possible modes of travel, how the geography of an area affects the movement of people, etc.)
Especially for large countries such as the United States, Canada, Australia, there can be plenty of things to research which do not involve finding explicit emigration/immigration records. If we cannot use migration to tag movement within country boundaries, what should we use instead?