When a company owns numerous brands within the same vertical it can spur marketplace confusion and wasted marketing dollars if the 'job descriptions' of each brand are not clear.

Brand Portfolio Strategy is the result of specifying the roles and relationships of a company's brands to one another to ensure they are clearly positioned and clearly marketed to the company's target audiences.

Brand Portfolio Strategy is distinguished from Brand Architecture in that Brand Architecture is the structure of an individual brand. 

Brand Portfolio Strategy is also be referred to as Portfolio Managment Strategy, Portfolio Architecture.

Suggested Reading:

Brand Portfolio Strategy: Creating Relevance, Differentiation, Energy, Leverage, and Clarity author: David A. Aaker asin: 0743249380