Users tend to be concerned with what a system does (features, functionality) and sysadmins tend to be concerned with the operational aspects of a system. I just noticed this great Wikipedia page that lists “Non-functional requirements” of a system.
Broadly, functional requirements define what a system is supposed to do whereas non-functional requirements define how a system is supposed to be. Functional requirements are usually in the form of “system shall do “, while non-functional requirements are “system shall be “.
I could see myself using this as a tool for jogging my memory when I’m trying to think of all the aspects of a system that I need to be concerned with either operationally or when writing requirements.