Same as term Administrative Services Only: services provided in an employee benefit plan such as a pension plan. An employer provides the clerical staff to operate the plan, in effect acting as custodian. The trustee provides direction for investment of the plan's funds, usually in a self-directed investment account. Trustee plans are gaining in popularity as both the employer and employees seek more control over pension funds investments. In a self-insured property or liability plan the group may have an ASO contract with an insurance company or a third-party administrator to handle claims processing and administration.