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

In radio frequency allocations, services are designated as either 'primary' or 'secondary'. When the Amateur Radio Service is secondary on a band, amateurs are allowed to use the frequencies, but they must yield to primary users. This means you must not cause harmful interference to primary stations, and you must accept any interference from them. The incorrect answers focus on foreign amateurs, international communications, or digital modes. A secondary status solely dictates the priority of services regarding who must yield to whom, and has nothing to do with restricting specific emission types or international contacts.