In the "Define IT Universe" stage of the IT audit plan development process, the primary action is to identify significant applications that support the business operations. This step involves mapping out the IT environment, including key systems, applications, and infrastructure components that are critical to achieving business objectives. By identifying these significant applications, auditors can focus on areas that have the greatest impact on the organization's performance and risk profile. This stage sets the groundwork for subsequent steps such as risk assessment, ranking subjects, and selecting audit engagements.
Reference:
IIA Global Technology Audit Guide (GTAG): "Developing the IT Audit Plan" ISACA's COBIT Framework for IT Governance and Management