Verification activities take place throughout the SLC and consist of specific checks of individual components or documents.
Audit activities take place throughout the SLC and confirm that functional safety procedures are being applied.
Validation is a specific SLC step, taking place after commissioning, but before introduction of hazards. Validation confirms that the installed SIF/SIS are working correctly in line with the SRS.
FSA confirms correct design/operation in line with functional safety requirements, but also makes a statement as to whether the requirements themselves are correct.
Increasing level of confirmation of functional safety compliance is as follows: Verification -> Audit -> Validation -> FSA