The project manager does not have to be the product/industry subject matter expert, but requires good understanding or knowledge of the industry and/or product of the project; some past experience will be highly desirable. This will help the project manager to ask relevant questions, coordinate better, understand and respond to issues faster and engage the team in a timely and effective way.
Some level of understanding or knowledge of the product/industry is very valuable, sometimes necessary, for successfully delivery of projects. The product subject matter expert is usually responsible for the product delivery management or a sub-set of it.
Examples of product/industry include engineering (electrical, mechanical, civil, chemical, and architecture), information technology, accounting/finance, human capital management among others and their specialized areas such as road construction, power engineering, computer software, data management, commercial banking, pharmaceuticals etc.