soap headers
I think what you're referring to was written to more or less identify what happens at the web service, not the client for the web service. Most frameworks/containers will spawn as many as 10 or more instances of a web service in order to handle concurrent requests, this also implies that any handlers associated with the service will also be created. The jsr statement is saying that it's possible for only one instance of the handler to be created and thus serve all of the inbound service requests etc.
Regardless, of how many copies of the handler that are, each invocation has its own message context which is unique to each client proxy instance. As long as you don't rely on private variables in your handler and store state information in the message context, then you're good to go