IEC61511-1:2016, § 3.2.25 defines functional safety audit as: systematic and independent examination to determine whether the procedures specific to the functional safety requirements comply with the planned arrangements, are implemented effectively and are suitable to achieve the specified objectives. Note: A functional safety audit may be carried out as part of a FSA.
The purpose of FS-Audit is to determine if functional safety procedures are being properly applied during project execution (think of it as an ISO-9001 audit for functional safety). The audit aims to identify potential systematic failures in the implementation of FS tasks and activities. In contrast to FSA, audit does not give a statement as to whether the required level of functional safety has been achieved; it only confirms that the correct procedures are being implemented as a prerequisite.