Defining the difference between a functional and a non-functional requirement:
A functional requirement is typically phrased as a noun/verb, e.g., “The system prints statements”
A nonfunctional requirement is a adverb or modifying clause, e.g., “ The system prints statements with confidentiality”.
From a mathematical point of view, a functional requirement takes an input and yields an output.
Non-functional refers to the manner in which such functions are preformed.
In you requirements document, you should have both functional and non-functional requirements, but they should be kept in different sections of the document. Sometimes they are in a separate Tech specs document, it just depends on how your organization documents them.