If information is identified that would be prejudicial to a positive outcome on your visa application, in most instances INZ are obligated to write to you and provide a reasonable opportunity for you to respond before making a decision. This is called a PPI (Potentially Prejudicial Information) letter.
PPI letters differ from simple requests for further information and need to be treated seriously, as they indicate that unless the concern of the Immigration officer is addressed adequately, a decision to decline the application may (will) be made.
If you have any doubt as to your ability to address such concerns, seek professional assistance as early as possible. Often individuals will respond to INZ with information or documents, but without supporting submissions or explanation, wrongly thinking INZ are only requesting information, and are then shocked to find their application declined. Often this can be avoided with a fully considered and detailed response