This library provides base classes for working with the legacy SharePoint client.svc/ProcessQuery endpoint. The base classes support most of the possibilities for types of query calls, as well as supporting fluent batching and caching. They are based on the same @pnp/queryable foundation as the other libraries so should feel familiar when extending. You can see @pnp/sp-taxonomy for an example showing how to extend these base classes into a functional fluent model.
Graphical UML diagram of @pnp/sp-clientsvc. Right-click the diagram and open in new tab if it is too small.