ID variables ;

The first variable listed in the ID statement identifies observations in the input and output data sets. Usually, the first ID variable is a SAS date or datetime variable. Its values are interpreted and extrapolated according to the values of the INTERVAL= option. See the section Data Periodicity and Time Intervals for details.

If more than one ID variable is specified in the ID statement, only the first is used to identify the observations; the rest are just copied to the OUT= data set and will have missing values for forecast observations.