Document classes have different process flows, or different states and events. They are treated as different types of objects.
The last two of the five document classes differ from the first three classes:
Documents are all the written stuff going in and out of projects during the life of a project.
In PRINCE2 / MSP vocabulary, documents in cattaDoc both comprise specialist products, ie. products or documents defined as project deliverables in the product-based planning process, and other documents flowing as part of the project processes, like agendas, minutes, e-mails etc.
Management products are specific document types specified by PRINCE2 for projects and by MSP for programmes.
Project registers and logs can be created in cattaDoc, whereas the rest of the management products can only be uploaded, ref. "create or upload", at least currently. Additional management products might be made available for creation in cattaDoc in the future, probably starting with product descriptions and quality logs.
A meeting is a "document" class where you can record meetings, telephone conversations etc. without creating a proper document.
Meetings are important events in projects and is often vital to record in themselves as well as in their relationships with persons, ie. which persons participated in a given meeting.
Library files are anything you choose to register so you can find them again and use them at some point in time. They can be digital files - like a piece of software, a code element, a manual - and they can be physical things you wish to register, as a reference.
As a default, library files have no relations to projects. But library files can be background documents for one or more projects. Examples of background documents are: