A means 65, 'B means 66, and so on. Number Constants as distinguishes three kinds of numbers according to how they are stored in the target machine. Integers are numbers that would fit into an int in the C language. Bignums are integers, but they are stored in more than 32 bits. Flonums are floating point numbers, described below.
Integers have the usual values.
The distinction is made because in some places integers are permitted while bignums are not. A flonum represents a floating point number. A letter, to tell as the rest of the number is a flonum. Case is not important. One or more decimal digits.