What's the difference between a requirements specification and a requirement?

The structure is known by its top-level requirements specification and a requirements specification is always the top level of a structure.

A requirement is a detailed statement or information, with different attributes than a requirements specification. A requirement can have multiple child requirements, like a requirements specification, but a requirement is never found at the top level of a structure.

A requirements specification object can be included in the structure of another requirements specification. This behavior is the same as a requirement. When a requirements specification is added to a structure, it can't have any child requirements.