BRD Versus MRD
Question: "Can you please explain the differences between a BRD and an MRD with respect to both new products and product enhancements? Is a BRD the same as a Business case?"
Answer: The "Business Requirements Document" or "Business Requirements Definitions" (BRD) is a document concept, in the context of project management and product development, that first appeared during the mid to late 1990's and did not achieve widespread acceptance in the technology industry.
The BRD document concept has several acutely diverse variations. The most common variation of a BRD outlines the details of a specific project for a particular customer, documents the customer's present needs, and notes all customer expectations from the proposed solution. Occasionally this type of BRD will contain a technical feasibility assessment and financial information. This BRD variant is regarded as being somewhat synonymous to another document concept that is titled "Project Scope Document" (PSD).
Some versions of BRD documents heavily focus on user requirements and this type of BRD is regarded as being closer to the document concept of a "System Requirements Document" (SRD) or "Functional Specification Document" (FSD).
The BRD, PSD, SRD, FSD, and SRS document concepts did not achieve proper consolidation or maturity and eventually were abandoned in favor of the familiar core set of product management documents that most market-driven companies use today: Business case, "Market Requirements Document" (MRD), and "Product Requirements Document" (PRD).
With regard to your second question, a Business Case is an examination of a potential market opportunity on a product level and an MRD is a description of the market opportunity, market problem and the resulting market requirements. Both these documents are very different than a BRD.