From OpenSCADAWiki
Simple asynchronous acquisition mechanism
The mechanism is characterized by requests to the data source, regardless of the request to the parameter attribute (Fig. 3). Usually, requests to the data source are made periodically in own polling task of the single taken controller and with the blocks of several signals. This request to the parameter attribute returns the value obtained from the last connection session with the data source. This mechanism is usually used when working with remote (network) data sources, characterized by high latency, i.e. delay in the response to the request.