Functional Requirements include:
1. Descriptions of data to be entered into the system
2. Descriptions of operations performed by each screen
3. Descriptions of work-flows performed by the system
4. Descriptions of system reports or other outputs

→ The Functional Requirements Specification is designed to be read by a general audience. Readers should understand the system, but no particular technical knowledge should be required to understand the document.

Examples:
1. Interface requirements
2. Business requirements
3. Regulatory/Compliance requirements
4. Security requirements

Software characteristics are
● Functionality: Refers to the degree of performance of the software against its intended purpose.
● Reliability: Refers to the ability of the software to provide desired functionality under the given conditions.
● Usability: Refers to the extent to which the software can be used with ease.
● Efficiency: Refers to the ability of the software to use system resources in the most effective and efficient manner.
● Maintainability: Refers to the ease with which the modifications can be made in a software system to extend its functionality, improve its performance, or correct errors.
● Portability: Refers to the ease with which software developers can transfer software from one platform to another, without (or with minimum) changes
● Robustness refers to the degree to which the software can keep on functioning in spite of being provided with invalid data
● integrity refers to the degree to which unauthorized access to the software or data can be prevented

Consistent :
The Software Requirement Specification(SRS) is said to be Consistent if and only if no subset of individual requirements described in it conflict with each other.
Unambiguous :
SRS said to be unambiguous if there exist no word that have more than one meaning and this will not confuse testers to get the exact reference.
Correct :
SRS is said to be correct if it is error free and accurate.

Maintenance:
Maintenance can be referred as a process of enhancement in the software product according to the changing requirements of user.
4 types of maintenance:
1. Adaptive – modifying the system to cope with changes in the software environment (DBMS, OS).
2. Perfective – implementing new or changed user requirements which concern functional enhancements to the software.
3. Corrective – diagnosing and fixing errors, possibly ones found by users.
4. Preventive – increasing software maintainability or reliability to prevent problems in the future.

The software requirements specification document is a requirements specification for a software system, is a complete description of the behavior of a system to be developed and may include a set of use cases that describe interactions the users will have with the software. In addition it also contains non-functional requirements. Non-functional requirements impose constraints on the design or implementation (such as performance engineering requirements, quality standards, or design constraints)
An SRS document should clearly document the following aspects of a system: Functional Requirements, Non-Functional Requirements and Goals of implementation

● Software Reliability is the probability of failure-free software operation for a specified period of time in a specified environment.
● robustness is the ability of a computer system to cope with errors during execution[1and cope with erroneous input
● Fault tolerance is the property that enables a ​ system​ to continue operating properly in the event of the failure of some (one or more faults within) of its components.
● Portability in high-level computer programming is the usability of the same software in different environments.

The system is considered as a black box whose internal details are not known that is, only its visible external (input/output) behavior is documented.