Interesting book about portfolio development

As more and more educators use portfolios, they increasingly recognize that the process has the power to transform instruction. Some teachers, however, are confused by the many types of portfolios, their different uses, and the practical issues surrounding storage, ownership, and the like.
The three major types of portfolios are: working portfolios, display portfolios, and assessment portfolios. Although the types are distinct in theory, they tend to overlap in practice. Consequently, a district's program may include several different types of portfolios, serving several different purposes. As a result, it is important for educators to be clear about their goals, the reasons they are engaging in a portfolio project, and the intended audience for the portfolios.
Working Portfolios
A working portfolio is so named because it is a project “in the works,” containing work in progress as well as finished samples of work. It serves as a holding tank for work that may be selected later for a more permanent assessment or display portfolio.
A working portfolio is different from a work folder, which is simply a receptacle for all work, with no purpose to the collection. A working portfolio is an intentional collection of work guided by learning objectives.
Purpose
The major purpose of a working portfolio is to serve as a holding tank for student work. The pieces related to a specific topic are collected here until they move to an assessment portfolio or a display portfolio, or go home with the student. In addition, the working portfolio may be used to diagnose student needs. Here both student and teacher have evidence of student strengths and weaknesses in achieving learning objectives, information extremely useful in designing future instruction.

From the introduction of the book.