Doing this specifies case conversion. Letters and digits. Printable characters, not including space. Printable characters, including space. Horizontal or vertical whitespace.
Equivalence classes are a recent invention intended to support non-English alphabets.
But there seems to be no standard way to define them or determine their contents. Characters not in set1 are passed through unchanged. When a character appears more than once in set1 and the corresponding characters in set2 are not all the same, only the final one is used. A common use of tr is to convert lowercase characters to uppercase. This can be done in many ways.