Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote sync, hoping to talk to another machine, and rush told you there is no such verb. Here is the idea behind that. In rush, message is the verb, full stop. What varies is the scope. To message a thread on the same machine, you give its address. To message a thread on another machine, you give the address of a thread that lives on another node: the same verb, a wider scope. So there is no separate sync, and no family of node verbs. The word sync was retired even before it was ever created, which is why you will not find it in older scripts. Rush says so by name, instead of claiming not found. The fix is to use message, with the destination being the node's thread, and with two way if you want an answer. One verb, and the destination decides how far the letter travels. If you defined your own function called sync, it still wins in scope. Say it with message, and let the scope do the rest.
Text from the Rush engine's error catalogue.