The Number of steps to forecast field allows the user to specify how many time steps into the future the model will produce predictions for. In this example we have entered "24" in order to get a monthly forecast out to 24 months beyond the end of the incoming priming data.
The Number of historical rows beyond end of training data field only becomes enabled when the step detects that the loaded forecasting model is using an artificial time stamp (see Section 3.1.2 of Time Series Analysis and Forecasting with Weka). In this case, the user can specify how many rows of the incoming priming data occur after the most recent row seen by the forecaster when it was trained - this enables the forecaster to synchronize the artificial time stamp value with the priming data.
The Rebuild/reestimate forecaster on incoming data check box allows the user to specify that the forecaster should be trained on the incoming data rather than primed. This allows the forecasting model to be brought up to date with the latest historical data. After training is complete, a forecast is generated as described above. Selecting this option enables the Save forecaster field. This field can be used to specify a file to which the updated forecasting model will be saved out to. Leaving the field blank tells the step not to save the updated forecasting model.