Understanding Requirements - Requirements Engineering and Management for Software Development Projects - page 23

Thus, there are three layers before the actual fabrication/manufacture/con-
struction begins and these are:
1. The needs
2. The product Specifications
3. The design
In software development too, we have three levels before the coding begins.
There are various nomenclatures for these three levels. I am presenting a few here
but there could be others.
1. User Requirements Specification (URS), System Requirements Specification
(SyRS), Business Function Specification (BFS), Functional Specification (FS),
Requirements
2. Low Level Design (LLD), Software Requirements Specification (SRS), Func-
tional Design Specification (FDS), Architecture
3. High Level Design (HLD), Software Design Description (SDD), Software
Design Specification (SDS), Detail Design Specification (DDS)
Now the next question that arises is ''what requirements do we need to man-
age—user requirements or software requirements?''
User requirements are original and first in the chain. Software requirements are
not original. They are derived from user requirements. When user requirements
change, the software requirements also change. Therefore, we need to manage user
requirements. If we can minimize changes to user requirements, the changes to
software requirements would automatically be minimized. This topic focuses on
managing user requirements.
2.6 Definition of Requirements in the Context of Software
Development
How do you answer the question ''what constitutes a software application?''
I am sure there will be numerous or multiple alternative answers for this
question. The answer I select is that the application consists of a number of
information processing processes. Information processing processes can be further
divided into three classes, namely,
1. Input processes
2. Output processes
3. Associative processes
Input processes obtain information from outside the application boundary. The
information would be provided by an entity (an individual, a machine or another
computer application). The input can be data (facts, and figures about an entity),
control data (triggers for events in the application such as start, stop, change, print etc.).