Recommended minting hierarchy
Parent	Primary child	Secondary child	Use child	Keyring child
Owner	Yes	Normally through PA	Normally through PA	Normally through PA
Primary Author	Optional	Yes	Yes	Yes
Secondary Author	No	Optional	Yes	Yes
Use Key	No	No	Optional narrower Use	No
Keyring Key	No	No	No	Optional narrower Keyring

For the MVP defaults:

PA → PA: disabled unless Owner explicitly enables it.
SA → SA: disabled unless explicitly enabled.
Use → Use: disabled.
Keyring → Keyring: disabled.
No upward minting is ever allowed.
A Use Key can never mint an Author Key.
A Keyring Key can never mint an Author or Use Key.