You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A review on integrated process planning and scheduling 
Chapter Title:  Brief review of process planning 
1. Abstract of Cited Paper (BIB001):  In recent years, a few researchers have addressed the need for the integration of process planning and scheduling functions in order to achieve superior overall system performance. Many of these researchers have discovered that the potential savings are substantial when process planning and scheduling are integrated. It has been reported that typical scheduling objectives, such as minimizing makespan, maximizing equipment utilization, etc, can be significantly improved as the result of integration of these two important manufacturing system functions. In this paper, we present a review of the reported research in this area, discuss the extent of applicability of various approaches, and suggest directions for future research. 
2. Abstract of Cited Paper (BIB002):  Agent technology has been considered as a promising approach for developing process planning systems and optimizing process plans in a distributed environment. A number of researchers have attempted to use agent technology in process planning for the purpose of integrating systems, tools and algorithms which are geographically distributed. In this paper, the historical background of agent technology is systematically reviewed. A comprehensive review of agent technology and its applications in manufacturing is presented including the applications of agent in scheduling, control, and enterprise integration and supply chain management. The focus of the paper is on how agent technology can be further developed in the support of collaborative process planning as well as its future research issues and directions in process planning. The current research directions in computer aided process planning (CAPP) and issues in traditional CAPP approaches are also discussed. Three agent-based approaches for process planning are presented. Finally, the key issues in developing agent-based collaborative process planning systems are summarized.

Literature Review:
A process plan specifies what raw materials or components are needed to produce a product, and what processes and operations are necessary to transform those raw materials into the final product. It provides the information to the shop floor on how to produce the designed products BIB002 . The outcome of process planning is the information required for manufacturing processes, including the identification of the machines, tools and fixtures. Since  proposed the method that used computer to aid the selection of process, there have been numerous research efforts in the development of Computer-Aided Process Planning (CAPP) systems. Many systems and many implementation approaches have been reported. Some earlier works of the CAPP systems had been summarised in BIB001 . It had reported about 187 CAPP systems. There are two traditional types of approach to CAPP: variant approach and generative approach BIB003 . The variant approach uses group technology to retrieve an existing plan for a similar part and makes the necessary modifications to the plan for the new part. This method is especially suitable for companies with few product families and a large number of parts per family. One of the main disadvantages of this method is that the quality of a process plan depends on the knowledge background of the process planners BIB003 . The generative approach is based on generating a plan for each part without referring to existing plans. It uses a more powerful software programme to develop a process plan based on the part geometry, the number of parts and information about facilities in the plant. Generative approach mostly meets the requirements of large companies and research organisations, especially those that have a number of different products but each product type is in small production batches. However, there are still difficulties in developing truly generative CAPP systems to meet industrial needs BIB003 . Most current CAPP systems use these two approaches.