The constant term in an ARIMA equation introduces deterministic trend into the model and extends that trend indefinitely into the future. If the model includes a single difference (either non-seasonal or seasonal) and the constant term is present, the trend is linear, where a twice-differenced model has quadratic trend. The AutoSelect setting for ARIMA constants in the Predictor ARIMA Options dialog leaves out the constant term in the model if it contains one or more non-seasonal or seasonal differences.
When the constant term is included in the model, the value of the term is calculated by the following equation:
Where φi are non-seasonal AR coefficients, Φi are seasonal AR coefficients, and μ is the mean of the series.