Simple enough. If a liege is using a succession law that is not otherwise available to a vassal, their vassals should have access to that succession law as well. This would only be for succession laws unlocked through innovations, not laws given to specific titles like primogeniture for Austria or Byzantium. So, if Bohemia blobs to their east, any of their vassals would have access to House Seniority as long as Czech culture as unlocked Table of Princes.
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