BIS524 Chapter 1

Example 10: Discussion
The Input statements in example 10 are different:

DATA MIXED;  
   INPUT @20 TYPE $1. @;  
   IF TYPE = '1' THEN  
      INPUT ID     1-3 ...

The first Input statement is an example of an Input statement with a "trailing @ sign." The trailing @ tells SAS to not read a new record the next time an Input statement is executed in this iteration of the data step. It effectively holds the record so that it can be reread by another Input statement.
So, the logic of this data step code is:

Read the part of the record that can tell us which kind of record it is.
Decide which type it is.
Depending on which type it is, use one Input or the other to correctly re-read the whole record.

Note also that the other Input statements (other than the first one) do not have a trailing @ sign. This releases the data record so that you are ready to finish the data step.
That is one of the two new things in this example. The other is the IF-THEN/ELSE statement.