If the regulations say that the amateur radio service is a secondary user of a frequency band, and another service is a primary user, what does this mean?

In radio regulations, frequency allocations are often divided between primary and secondary users. A primary service has priority on that band, meaning they have the right to operate without interference from secondary services. Secondary users, like amateur radio operators in certain shared bands, are permitted to use the frequencies but with strict conditions. They must not cause harmful interference to primary users and must accept any interference received from them. The distractors suggesting emergency-only use or brief testing are incorrect. Secondary use is intended for normal amateur operations, provided the strict non-interference rule is followed.