Possible Clues:

• Widely applied code that can't be read
• Oral has Bill succeeding with 'Custom and Practice'
• Organisation summons more than nine from the bar, as it's the custom and everyone agrees with it
• Organisation summons more than nine from the bar as it's the custom and everyone agrees with it
• Custom-based rule
• From word of mouth Bill gives rules of jungle perhaps
• As a rule, is it spoken?