The audit criteria define what the “rules” are. For the sake of this guide, the criteria will be the elements of ISO 14001. A subtle point to note however is that the site’s EMS requirements are also part of the criteria. This means that in addition to responding to the requirements of ISO 14001, the EMS must also respond to “planned arrangements”, or what the organization said it was going to do. In audits, a common response is “the standard does not require such and such detail”.
However, if the site’s procedure does require some specific response, then it becomes part of the criteria. In essence, the auditors are verifying the system not only to ISO 14001, but also to what the EMS documentation states. How the audit is divided and scheduled throughout the time interval is up to the organization and will be a function of minimizing disruption to site operations and resource needs. The only requirement is that the full audit be completed within the frequency established in the procedures under 14001, 4.5.4. One of the requirements regarding frequency is that how often an area is audited be in part a function of prior audit results. This means that the planned frequency may change with time based on what auditors are finding.
Read more on ISO 14001 Standard at http://www.iso9001-standard.us