"Specifications" refer to a contractual document describing what is expectd of the contractor by the client.
It is a document that describes, as precisely as possible, in simple terms, the needs that the contractor must meet. To the extent that only the contractor is really competent to propose an appropriate technical solution, the specifications should preferably describe the need in functional terms, independently of any technical solution, except to specify the technical environment in which the requested solution has to fit. The document, on the one hand, assures the client that the deliverables conform to what is written, and, on the other hand, prevents the client from changing its wishes over the course of the project and requesting new functionalities the contractor that were not initially provided for.
Specifications must also contain all information necessary for the contractor to judge the size and complexity of the project in order to be able to make the most appropriate offer possible in terms of cost, timeframes, human resources, and quality assurance.
In this regard, it is a reference document that eliminates all ambiguity as to expectations, as well as a tool for dialogue that allows the contractor to ask questions of the client in order to refine its understanding of the client's demands. However, specifications are not necessarily static. Its content can certainly be modified over the course of the project, even if the ideal is to have everything laid down at the beginning on the basis of an addendum accepted by both parties.
Some projects fail due to miscommunications and, in particular, a lack of common culture and vocabulary between client and contractor. In fact, where the client believes it is using a generic term, the contractor will sometimes understand it as a technical term with a specific meaning.
The schedule desired by the client must be very clear and explicit and show the date on which the project absolutely must be finished. Ideally, the milestones will be specified in order to avoid a "tunnel effect".
Specifications are a contractual document signed by the contractor and the client, generally has a certain number of legal provisions that determine, for example, who possesses the intellectual property rights to the project, the penalties in case of breach of deadline provisions, or the courts to which any litigation will be submitted.