Exploration 17 introduced and discussed characters. This Exploration continues that discussion with character classification (e.g., upper or lowercase, digit or letter), which, as you will see, turns out to be more complicated than you might have expected.
As you learned in Exploration 17, the numeric value of a character, such as 'A', depends on the character set. The compiler must decide which character set to use at compile time and at runtime. This is typically based on preferences that the end user selects in the host operating system.
Character-set issues rarely arise for the basic subset ...