1. Determining the technical field is essentially determined by technical issues, not by the actual application field of the patent.
For example, a square container can be used in the field of office supplies to hold drinks, in the field of toiletries to hold bath liquid, and in the field of smelting to hold molten metal. From a technical point of view, the first two applications solve the same problem, which is to hold ordinary liquids, so they belong to the same technical field, while the last application is to hold special liquids, which has a negative impact on the material and strength of the container. have special requirements and therefore belong to different technical fields.
2. In the same technical field and the same technical solution, you can only apply for one patent; in different technical fields, because the problems solved are different, although the technical solutions have similar structures and principles, they must have special features. Therefore, different patents can be applied for. This is just like using the two wings of an airplane on a submarine. Because the environment in which it is used has changed, the problems it solves are naturally different. It will definitely be different from when it is used on an airplane, so you can apply for different patents.