How are US amateurs restricted in segments of bands where the Amateur Radio Service is secondary?

In the complex world of RF spectrum management, frequencies are often shared among different types of services. When the Amateur Radio Service is allocated as a 'secondary' user on a band, it means amateurs share the spectrum with a 'primary' service (such as radiolocation or industrial users) but have a lower legal priority. As a secondary user, an amateur station is legally allowed to transmit but must actively avoid causing harmful interference to any primary service stations. Furthermore, the amateur station must accept any interference it receives from those primary users. The distractors involving digital modes or international contacts are incorrect because priority status regulates interference and sharing, not emission types or contact locations.