salt-api is a modular interface on top of Salt that can provide a variety of entry points into a running Salt system. It can start and manage multiple interfaces allowing a REST API to coexist with XMLRPC or even a Websocket API.
Each netapi module will have differing configuration requirements and differing required software libraries.
Exactly like the various module types in Salt (execution modules, renderer modules, returner modules, etc.), netapi modules in salt-api will not be loaded into memory or started if all requirements are not met.
The core functionality for salt-api lies in pluggable netapi modules that adhere to the simple interface of binding to a port and starting a service. salt-api can manage one or many services concurrently.