Что означает `st1` и `st2` в данном примере?

Finding Rows with the Maximum Value of a Column by Group

This example returns the best test results of each student:

SELECT name, test, score FROM student_tests st1 WHERE score = (
  SELECT MAX(score) FROM student st2 WHERE st1.name = st2.name
); 
+---------+--------+-------+
| name    | test   | score |
+---------+--------+-------+
| Chun    | SQL    |    75 |
| Esben   | SQL    |    43 |
| Kaolin  | Tuning |    88 |
| Tatiana | SQL    |    87 |
+---------+--------+-------+

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