A more thorough explanation:

A looseleaf service is a type of lawbook that is designed to cope with constant change and increasing bulk. It has pages that are periodically replaced with updated pages. The first looseleaf service covered the federal income tax and was published in 1913 shortly after the Federal Income Tax Law of 1913 went into effect. It was followed in 1914 by a service reporting on the activities of the Federal Trade Commission, which had just been established.

The looseleaf method was first used as a means of reporting new tax and business laws which were to be subject to administrative interpretation. These first looseleaf services were designed not to reprint just the bare text of the revenue and commission acts, but to follow up and report each new development on these new laws as it occurred.

For example, a looseleaf service on employment law may have a section on discrimination. If there is a new court case that changes the interpretation of discrimination law, the looseleaf service will update that section with the new information. This allows lawyers and other legal professionals to stay up-to-date on changes in the law without having to purchase an entirely new book every time there is an update.