The function INTMAX returns the constant 10737421823 = 2^30-1. Although SuperBASIC’s integers can only handle a maximum of 16 bits (resulting in a range of 216 different values: -32768 to 32767), the QL can internally, on a machine code level, happily handle larger integers. Many keywords listed in this book actually make use of this possibility and that explains their valid parameter range.
An error in the Maths Package (up to v2.04) means that a line such as PRINT 2^30-INTMAX will report an out of memory error. This is fixed in later versions of the package.