The nature and design of each unit process accommodate the physical and chemical characteristics of the particular materials or types of materials for which each is intended. Because most discarded materials have certain characteristics in common, basic processing principles (e.g., those pertaining to size reduction, air classification, and screening) usually are non-specific with respect to the materials in a category. However, the details of equipment design, size, degree of complexity, and cost of individual unit processes are strongly influenced by the nature and utility of the material to be recovered, and the extent and degree of source separation. The gradation also depends upon the function being served, e.g., recovery from the waste stream, readying the recovered material for reuse.