The Web Api service is a http service that can be remotely called to execute a project.
The appRulesPortal.WebApiService service is normally created by the appRulesPortalSetup exe.
If not, you can manually install it running 'As administrator' the related cmd located in the appStrategy\PortalData\Services_Installation folder (install_webapi_service.cmd) You can also run [Install_path]\appRulesPortal\bin\appRulesPortal.WebApiService.exe -i to install (or -u to uninstall)
The http address and port of the webapi is indicated in the config file appRulesPortal.WebApiService.exe.config in [Install_path]\appRulesPortal\bin
You can also indicate an ApiKey for security purpose, the webapi will launch a project only if the client specifies it in the call.
see Using Web Api to learn how to use the webapi.