The following moves further along the spectrum from user needs to system
characteristics
Requirements are …a specification of what should be implemented. They
are descriptions of how the system should behave, or of a system property
or attribute. They may be a constraint on the development process of the
system (Sommerville and Sawyer 1997)
What - A property that must be exhibited by a system in order to solve a
real world problem
How - Product Parameters
Functional
Product Capabilities
Non-Functional
Constrain the solution
Constraints - Process Parameters
Constraint on the development process