rain_rate_last_mm = the rain rate (mm per hour) that was observed during the reporting interval
rainfall_mm = the depth of the rain collected (in mm) during the reporting interval
If you are asking about how often current conditions are transmitted from the station to the WeatherLink service that depends on the type of device you have. A WeatherLink Live sends data every 60 seconds and EnviroMonitor is based on the cellular service plan of 5, 15, or 60 minutes. But if you are asking about what data is available via the API that depends on the subscription level. This documentation page https://weatherlink.github.io/v2-api/data-permissions discusses what data is available.