TD_ARIMAXESTIMATE is used to fit the model that considers more than just past values. It suggests adding one or more outside (exogenous) variables to measure an inside (endogenous) variable. After finalizing the coefficients of the model with exogenous variables, TD_ARIMAXFORECAST provides better forecasting on the endogenous variable that you are interested in with the future values of those outside factors.
The TD_ARIMAXFORECAST function works with the ARIMAX model estimation function TD_ARIMAXESTIMATE. The function uses the ARTMODEL layer of the ART (analytic result table) from TD_ARIMAXESTIMATE, and exogenous variables that are equal to the number of steps ahead that endogenous variable when using EXOG_IMPUTE (NONE). There are several methods to determine the exogenous variables, such as using an univariate forecasting model like mean value of the samples.
The TD_ARIMAXFORECAST function considers exogenous variables during the forecast but does not forecast exogenous variables future values.