What a font role is set to.
"Follow the app default" is a state, not a value. The alternative - writing today's default into the file - froze the default forever: v1 files wrote every font key on every save, and nobody saw a new default font until their file was deleted. So FollowDefault persists as no key at all, and a chosen family is always written even when it happens to equal the current default. That last part is a deliberate change from the pre-SP-6 writer, which omitted a key whose value matched the default and so could not tell a choice from a habit either.
