When you create a new data set by combining observations
from two or more data sets, knowing which data set an observation
came from can be useful. For example, you might want to perform a
calculation based on which data set contributed an observation. Otherwise,
you might lose important contextual information that you need for
later processing. You can determine which data set contributed a particular
observation by using the IN= data set option.
The IN= data set option enables you to determine which data sets have contributed to the observation that is currently in the program data vector. The syntax for this option in the SET statement follows: ...