A cookbook version represents a set of functionality that is different from the cookbook on which it is based. A version may exist for many reasons, such as ensuring the correct use of a third-party component, updating a bug fix, or adding an improvement. A cookbook version is defined using syntax and operators, may be associated with environments, cookbook metadata, and/or run-lists, and may be frozen (to prevent unwanted updates from being made).
A cookbook version is maintained just like a cookbook, with regard to source control, uploading it to the Chef Infra Server, and how Chef Infra Client applies that cookbook when configuring nodes.