Final answer:
Contractors are required to provide professional services as engineers or architects when they are involved in designing a building or structure, including specifying materials and ensuring compliance with regulations.
Step-by-step explanation:
Contractors are required to provide professional services that constitute the practice of an engineer or architect in specific instances. For example, when a contractor is involved in designing a building or structure, they are required to provide engineering services. This could include developing the structural design, specifying materials and construction techniques, and ensuring compliance with building codes and regulations. Similarly, if a contractor is involved in designing the layout and appearance of a building, they are required to provide architectural services. This could involve developing the building's design, specifying the layout of rooms and spaces, and considering aesthetic aspects.