Think of the casualty market as having a support crew. None of these organizations sells insurance. Each one supplies something that insurers and agents use every day.
Insurance Services Office (ISO): an advisory organization. It collects premium and loss data from insurers and publishes standard policy forms and advisory loss costs. Under Ins. Code §1855.5, an advisory organization may distribute forms, classification codes and prospective loss costs once they are submitted to the commissioner, but it may not set rates or issue a manual of final rates.
ACORD (Association for Cooperative Operations Research and Development): a nonprofit standards body. It publishes standard forms, such as applications and certificates of insurance, and electronic data standards. A certificate on an ACORD form only summarizes coverage; the promise to pay comes from the insurer's policy.
California Workers' Compensation Insurance Rating Bureau (WCIRB): California's workers' compensation rating organization. Under §11750.3 a rating organization collects statistics to develop pure premium rates (submitted to the commissioner), administers classification and rating rules, inspects risks, examines policies and endorsements, and may start test audits of payroll within one year after a policy expires. On the employer's request, after notice to the insurer, it may give the employer the rates that apply to its insurance. It does not write policies.
Other states use other rating organizations, for example the National Council on Compensation Insurance, Inc. (NCCI). California uses the WCIRB. Section 11750.3(g) lets a California rating organization exchange ratemaking data with rating organizations, advisory organizations and insurers in this and other states.