In order for an SAI to claim ISSAI compliance, it must follow the fundamental principles which INTOSAI has defined as universally applicable professional standards in ISSAI 100. Para 7 of ISSAI 100 establishes the fundamental principles which are applicable to all public-sector audit engagements, irrespective of their form or context. ISSAI 200, ISSAI 300 and ISSAI 400 builds on and further develops the principles to be applied in the context of financial, performance, and compliance audit respectively. ISSAI 100 states that the principles in no way override national laws, regulations or mandates or prevent SAIs from carrying out investigations, reviews or other engagements which are not specifically covered by the existing ISSAIs1.
The principles defined under ISSAI 100 can be used to establish authoritative standards in three ways:

As a basis on which SAIs can develop standards;
As a basis for the adoption of consistent national standards; and
As a basis for adoption of the General Auditing Guidelines as standards.