IEC61511-1:2016, § 3.2.86 defines functional safety validation as: confirmation by examination and provision of objective evidence that the particular requirements for a specific intended use are fulfilled. …this means demonstrating that the SIF(s) and SIS after installation meet the SRS in all respects.
Validation takes place after commissioning and is often seen interchangeably with the Site Acceptance Test (SAT), i.e. it confirms that installed SIF/SIS are working correctly in line with the SRS. Note: FSA-3 goes a step further and confirms that the requirements themselves are complete, consistent and sufficient to achieve the intended level of safety.