The most important factor when defining the IS audit scope is to understand the relationship between IT and business risks, as this helps to identify the areas that have the most potential impact on the organization's objectives, performance, and value. By understanding the IT and business risks, the IS auditor can focus the audit scope on the key processes, systems, controls, and issues that need to be assessed and addressed.
References
ISACA CISA Review Manual, 27th Edition, page 256
Ten Factors to Consider when Setting the Scope of an Internal Audit
What Is an Audit Scope? | Auditing Basics | KirkpatrickPrice