**2. FbD Teminology**

During FbD practice, specific terminology, both technical and non-technical, is usually used. Important terms have been compiled in **Table 2** to facilitate better understanding of FbD of oral DDS precepts. Prior to applying FbD, it is critical to be familiar with FbD terminology and have prior multidisciplinary knowledge on various possible products and process variables. Therefore, it is necessary to separate a "knowledge space," or a whole worth exploring area, from the potential large ocean of scientific material based on prior knowledge. As a result, a "knowledge space" includes all the product and process variables that could even slightly affect the final product's quality. A "design space" must be defined as a subset construct of a "knowledge space" to ensure the best possible performance of a process or product using a "chosen few" key variables. This "design space" is used to further derive the "control space," which is the experimental area reserved for in-depth research when studies are conducted within narrower ranges of input variables. It is also known as "control tactic" on occasion. The knowledge space is transformed into the control space using a methodical strategy on archived data in the "design space" [11]. For relatively complex DDSs, extensive experimentation may be required to eliminate uncertainty and justify a design space greater than that needed for traditional formulation systems like tablets. Working within the design space would not begin any post-approval change process in accordance with federal regulations because it is not regarded as a "change" [3]. **Figure 1** depicts the order of importance in the knowledge, design, and control spaces.
