## 2 Literature Review
### High-Complexity, Low-Volume Products, and Production  
The definition of complexity was defined by [PERSON] [33]: \"The complexity of a product can be attributed to an increase in the number of parts; number of systems needed to accomplish product functions; number of external systems affecting the product; types of technologies associated with the system; number of interfaces among the systems; number of variables associated with the systems and their interfaces; number and types of users and uses and variations in the operating environments and number of disciplines or specialized fields needed to analyze, design, and evaluate various components and systems\". A natural consequence of highly complex products is a more complex design process, which needs more design support [8]. Highly complex products are more common in low-volume than high-volume production. That is to say, high-volume production is often linked to simple products, whereas low-volume production is often linked to customized products [8]. [PERSON] and [PERSON] [8] summarized the characteristics of \"High-volume manufacturing of low-complexity products (high volume, low complexity, HVLC)\" and \"Low-volume and customized manufacturing of high-complexity products (low volume, high complexity, LVHC)\" from previous studies. HVLC consists of attributes of \"small and simple interchangeable parts\", \"standardization\", \"innovation process of product development to customer demand\", \"focus on manufacturability\", \"specialized tools and fixed machines\", and \"economies of scale\". LVHC consists of attributes of \"large and complex parts\", \"customization\", \"innovation process of customer demand to product development\", \"large machining center, common tools and flexible machines\", and \"fewer parts to share costs\" [39; 40]. Under the incremental manufacturing circumstances of Industry 4.0, it is easy to re-construct a flexible production line with a cyber-physical system to collect various information and realizewhat and how many product specifications could be customized. To augment the potential scenario, the authors consider that a configuration system under Industry 4.0 should meet the following requirements [8; 16; 26; 27]: (1) Directly collecting and meeting customer dynamic requirements. In a typical procedure, a customer discusses and negotiates through a sales interview and places an order with the assistance of sales office personnel. Sales departments consult and obtain technical advice or feedback from technical departments to verify the correctness and compatibility of the recommended configuration. Efficiency would be impaired by a communication barrier and subjective bias. However, the existing configuration system puts more emphasis on transforming customer requirements into manufacturing requirements for conventional high-volume manufacturing for technical personnel, as shown in Figure 1a. Instead of using mass requirements (HVLC) as input, the novel and quick-response product configuration system should be able to handle individual customer requirements (LVHC), as shown in Figure 1b. (2) Good interaction with customers. Customers do not know the details of the product configuration they really need. Hence, the quick-response product configuration system should be able to assist sales personnel in responding to customers to provide appropriate configurations by their particular customer preference. Figure 1 illustrates the roles of the conventional configuration system and the novel configuration system in two different contexts of high-volume manufacturing of low-complexity products (Figure 1a) and low-volume and high-complexity customization manufacturing under Industry 4.0 (Figure 1b). Hence, the authors intend to propose a methodology of designing a novel quick-response product configuration system to enhance the communication between the service demanders (customers) and the manufacturers.