A more thorough explanation:

Definition: The Index to the U.S. Patent Classification System is a tool that helps people understand and use the Patent Office's classification system. It includes about 65,000 common words and phrases and shows which class and subclass they belong to in the system.

Example: Let's say you have an invention for a new type of bicycle. You can use the Index to the U.S. Patent Classification System to find the right class and subclass for your invention. You might look up the word "bicycle" in the Index and find that it belongs to Class 280, which is for land vehicles. Then you might see that bicycles specifically belong to Subclass 281, which is for two-wheeled vehicles.

Another example: If you have an invention for a new type of computer mouse, you can use the Index to find the right class and subclass for that too. You might look up the word "mouse" and find that it belongs to Class 345, which is for computer graphics processing and selective visual display systems. Then you might see that computer mice specifically belong to Subclass 156, which is for cursor positioning control device.

These examples show how the Index to the U.S. Patent Classification System can help inventors find the right class and subclass for their inventions. This is important because it helps the Patent Office organize and search for patents more easily.