What are non-standard screws? Why are there non-standard screws?

What are non-standard screws? Threads that do not conform to our standards, other countries' standards, or international standards are considered non-standard screws. In line with our country's standards is in line with national standards; in line with British standards is in line with British standards; in line with U.S. standards is in line with U.S. standards; there are German standards; French standards; Russian standards; too much. Why there are non-standard screws?1, the limitations of the environment. For example: 10 × 1 pipe can be tapped inside the (M9 × 1) thread. 2, corporate behavior. For example: the locking bed of a household sewing machine is (M5 × 1) thread. There are so many reasons for the formation of non-standard, non-standard screws more. I hope my words can help you