Aha this one from jsr 224 cleared things up for me:
9.3.3 Handler Implementation Considerations
Handler instances may be pooled by a JAX-WS runtime system. All instances of a specific handler are
considered equivalent by a JAX-WS runtime system and any instance may be chosen to handle a particular
message. Different handler instances may be used to handle each message of an MEP. Different threads
may be used for each handler in a handler chain, for each message in an MEP or any combination of the
two. Handlers should not rely on thread local state to share information. Handlers should instead use the
message context, see section 9.4.