Priority defines what order the services will be run in. 1-9 are foreground services ordinarily executed before a response is returned to the user. a-z are background services run after a response is returned to the user. Two services sharing the same priority will be run concurrently (but see the config.app_config.threaded_services config param).
Services defined in services.yml may have other service-specific parameters too, for instance commonly a password or api_key giving you access to the foreign web service .