A more thorough explanation:

The key-number system is an indexing system used in legal research to organize American caselaw with headnotes. It was developed by West Publishing Company, now known as West.

In this system, a number is assigned to a specific point of law, allowing researchers to easily find all reported cases that address that particular point by referring to its number. For example, if a researcher is looking for cases related to the Fourth Amendment's protection against unreasonable searches and seizures, they can search for the key number 198 to find all cases that discuss this topic.

The key-number system is a useful tool for legal researchers because it allows them to quickly and efficiently locate relevant cases on a specific legal issue.