It can be whatever the writers of the NoR or SI want it to be based on the boats and expected local conditions.  For match and team racing arm signals are typically sufficient as the people you want to make sure know of the request/response are the umpires who have an engine right behind them and may not hear the hail.  This would also hold for 10-15m boats on a windy day, a summer day on San Francisco Bay for example.  For super-maxis it may be a VHF radio call as a hail is nearly impossible to hear and arm signals might be hard to see.  It could even be a light system installed on all the boats e.g. America's Cup boats.  The rule change is just to make it easier for the OA or RC to add a requirement.

John,

Many thanks