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FRS is the Functional Requirement Specification it is a intial phase of SDLC. Gathering the requirements from customer and prepares the Business Requirement specification by Business Analyst's.
Next in Analysis phase System Analysts analyze the BRS and prepare the SRS (System Requirement Specification).
BRD is a high level documention (means tables) and SRS/FRS is Low level documention (use case diagrams)
As per my concerned if this question will majorly asked during Interview processes or during any internal meetings, then should be answered in a sentence , also I appreciated the detailed information of "Indy"
BRS: Stands for Business requirement Specification which defines the requirement of the client to be developed.
FRS: Stands for Functional Requirement Specification which defines the architectural process of a particular requirement , while
SRS: stands for Software Requirement specification which defines the functional requirement to be developed and system requirement to be used.
While the basic difference in them is that the BRS comes under the HLD (High Level Design) and SRS and FRS will come under the LLD(Low Level Design) process of Designing Phase of SDLC process.
Also BRS comes under the SQA process of Development while FRS and SRS will come under the SQC process.
I want to add a point to get a clear understanding of what SRS is:
1st is software requirement specification
2nd is functional requirement specification
................................................NO DIFFERENCE..........................other then definations
Call it BRS, FRS, SRS, FRD, BRD......or call it by any other term.. Objective is same....Requirments are Discussed->Gathered->Listed/Mentioned....Reviews are done.....Final Doc is made which comes in as a final Draft for Further Process (Design->Development->Test)
People Involved are...Customer->BA->Develeopers&Testers (Including or excluding Team Managers and Process Review Teams)...that to depend upon organizations...no hard and fast rule.....
BRS: Business Requirement specification-It is a ruff documentation prepared by Business Analyst gather the information from Client.
SRS and FRS : System Requirement Specification and Functional Requirement Specification both are similar, here PM/PL prepares Technical Documentation with help of BRS.