A request is a piece of information that is sent to the server by a client. The client and the server have to agree on the protocol that they use to communicate, in order to allow them to interact with each other.

In order to facilitate the exchange of data, it is recommended that an SDK (or some sort of library) be provided by the product vendors. For example, if you're interested in interacting with a database from a Java application, there are drivers coded in the form of libraries that can be used for that. Furthermore, the database vendors also provide drivers for different programming languages, desktop applications, or UIs, to interact with servers such as pgAdmin or MySQL Workbench.

Providing an SDK is not a must; even when an SDK is provided, an easy to understand document will avoid introducing a conformist relationship between the server and clients.

Conformist relationship is a term coined by domain driven design. It suggests that a service has a complex and large model that forces the downstream dependencies to be modified when the server introduces new changes or releases new features. The modification should happen, because the effort required to write their own model adaptations or mechanisms to interact with the server is extremely high and is difficult to achieve.