Material causes are the resources the production of goods and services consume. The efficient causes are the transformative factors that serve as a fund for the production process.
Factors tend to substitute for each other other within the class of causes – material or efficient – and they tend to complement each other between the classes.
The covenantal issues comprehend the relation between the classes; the contractual issues comprehend the relation within the classes.
Trade-offs deal well with substitutes but they deal poorly with complements.