Wash. Admin. Code § 296-801-099
Staffing agency. An employer whose business is classified under the North American Industry Classification System (NAICS) 561320 industry sector. In general, these organizations recruit and hire employees and temporarily assigns those employees to perform work or services for another organization, under such other organization's supervision, to:
(a) Support or supplement the other organization's workforce;
(b) Provide assistance in special work situations including, but not limited to, employee absences, skill shortages, or seasonal workloads; or
(c) Perform special assignments or projects.
Worksite (also referred to as host) employer. An employer with which a staffing agency contracts or otherwise agrees to furnish persons for temporary employment in the industries described in NAICS sectors 23 (construction industry sector), and 31 through 33 (manufacturing industry sector).
Wash. Admin. Code § 296-801-099