Specifications define outputs and are created by the solutions development procedure. The structure and content of a specification is entirely dependent on the context. In construction a specification may comprise, layouts, elevations, bills of quantities, structural details and so on. In IT, a specification could be functional or technical.
Since stakeholders are interested in the progress of their deliverables it makes sense for deliverables to automatically be considered as configuration items to ensure that progress and quality information is readily available.
The specification is whatever is required by good practice in the relevant context. In many contexts, the specification for an output may be formed from a series of descriptions of individual products. Some of these products may be deliverables in their own right and some tracked as configuration items in a configuration management system.