The range or scope of something is its purview. If you are a professional race car driver, performing surgery is way outside your purview.
Anything inside a range of interest or activity is within a purview. A supervisor is in charge of certain workers — other workers are outside her purview. The surgeon general deals with medical issues: the defense budget is outside his purview. When a business expands, it's trying to increase its purview. If you go to the dentist and ask her to paint a picture of your tooth rather than extracting it, you're likely to discover you've gone way beyond her purview.