WAC 381-70-240
WAC 381-70-240. Service of papers—By whom served
The board shall cause to be served all notices, orders, and other papers issued by it, personally or by first class mail, together with any other papers which it is required by law to serve. Every other paper shall be served by the party filing it with the board with proof of service, either personally or by affidavit of service by first class mail upon the parties and all counsel.