This is from permissions+tips:

• in general, an attempt to update a card that doesn't exist will fall back to the create action and use the create permission.
• changing a card type requires permission to delete the old card and create the new
• one special case in inheritance is adding new children to an existing parent.  In the example above, this would be like editing an Audition later to add some missing information, like, say, the timeslot.  Since, in this case, creating the child is akin to updating the parent, then whenever the child is set to inherit update permissions, it inherits them from the parent's create permissions.
• the commenting overhaul blueprint proposes to get rid of the *comment setting, so that this will no longer be considered a separate permission.  (instead comments will have full CRUD permissions based on a +*comment card).