You use the ID statement to name the variables in the DATA= data set that are not involved in the model but are to be transferred from the input data set to the output data set.
Variables listed in the ID statement must be contained in the DATA= data set. They can also be contained in other input data sets. If an ID variable is also contained in an AUGMENT= or INITDESIGN= data set and an observation from that data set is used in the final design, the values of the ID variables for that observation are transferred to the OUT= data set. For details, see the section Input Data Sets.