Ask yourself (act like you are the SAS compiler):
Look again at the data step:

Data Grades;
     ID = _N_;
     Input Score;
     GRADE = Put (Score, Score.);
Datalines;

Where does the compiler first encounter GRADE?
Is GRADE a character or numeric variable?
What is the length of GRADE? (How many bytes of storage are used to store each value on disk?)
Normally FORMATS are specified with a field width (e.g. FORMAT SCORE 5.1; ) If a field width is not specified, a default width is used.
What value of w is used when SAS compiles

 GRADE = Put (Score, Scorew.);  ?

The output and log
The Print output shows a few interesting features.

Example 4

ID     SCORE    GRADE

 1      55.0      F
 2      65.0      D
 3      74.0      C
 4      76.0      C
 5      88.0      B
 6      92.0      A
 7      94.0      A
 8      96.0      A
 9      98.0      A
10       0.0      F
11      -1.0      *
12     101.0      A
13      82.1      B
14      89.1      B
15        .       .

The log explains the asterisk (somewhat). Below the data step, the following note appears.

NOTE: At least one W.D format was too small for the number to be
      printed. The decimal may be shifted by the "BEST" format.

We will cover more SAS functions later.