The X11 Procedure

ID Statement

ID variables ;

If you are creating an output data set, use the ID statement to put values of the ID variables, in addition to the table values, into the output data set. The ID statement has no effect when an output data set is not created. If the DATE= variable is specified in the MONTHLY or QUARTERLY statement, this variable is included automatically in the OUTPUT data set. If no DATE= variable is specified, the variable _DATE_ is added.

The date variable (or _DATE_) values outside the range of the actual data (from ARIMA forecasting or backcasting, or from YRAHEADOUT) are extrapolated, while all other ID variables are missing.