6.5..9 Auditing for Method Service Set defines:
All Services in this Service Set for Servers that support auditing may generate audit entries and shall generate audit Events for both successful and failed service invocations if the invocation modifies the AddressSpace, writes a value or modifies the state of the system
...
Methods that do not modify the AddressSpace, write values or modify the state of the system may generate events

6.5.8 Auditing for Attribute Service Set defines:
In particular, the Write Service shall generate an audit event of type AuditWriteUpdateEventType or a subtype of it.

This would require that every write of every value is required to trigger an AuditWriteUpdateEventType. This makes sense for setpoints written by a user but it makes no sense for values written for process data exchange e.g. cyclic write of data from another process from a headless client e.g. controller to controller communication.