Nordstrom, an upscale department store, has a well-known reputation for going the extra mile to serve its customers. This reputation for excellent customer service will most likely result in a sustainable competitive advantage.
When a firm offers its customers with any products that has higher quality and benefits or lower price that its competitors then it refer to the competitive advantage. It is the advantage that the firm attains when compared with their competitors.
There are many different kinds of competitive advantage that can be used by a firm such as cost, product differentiation, niche strategies, etc. It is the thing that is absent in the competitors. In the given example, Nordstrom has well known reputation for the service that it gives for its customers and hence it is the likely result of sustainable competitive advantage.