国内软件需求的书籍比较少，而且不是很全面。下面是我在美国看的软件需求的一本书上的对于软件需求的分层种类。希望对你有所帮助。
Requirements Level Classification（需求的分类）
To deal with the diversity in requirements types, Sommerville (2005) suggests
organizing them into three levels of abstraction:
User requirements
System requirements
Software design specifications
User requirements（用户需求） are abstract statements written in natural language with
accompanying informal diagrams. They specify what services (user functionality)
the system is expected to provide and any constraints. In many situations user stories
can play the role of user requirements.
System requirements（系统需求） are detailed descriptions of the services and constraints.
System requirements are sometimes referred to as functional specification or technical
annex. These requirements are derived from analysis of the user requirements.
They act as a contract between client and contractor, so they should be
structured and precise. Use cases can play the role of system requirements in
many situations.
Software design specifications（软件需求） emerge from the analysis and design
documentation used as the basis for implementation by developers. The software
specification is essentially derived directly from analysis of the system specification.
The software requirements specification document (SRS) is the “contractual” document
that we generally refer to when we speak of a “software” or “system” requirements
specification.In the case of a hybrid hardware/software system,（考虑软件和硬件系统） the SRS would include both detailed design elements for the software as well as hardware
(e.g., schematic diagrams and logic diagrams). In the case of a purely mechanical
system, design drawings take the place of the SRS.
