The chapter is about SQL functions. Notes are in chapter order. Scalar functions operate against a single value and return a single value based on the input value.
Aggregate functions return a single value based upon a set of other values.
The rules are somewhat more complex than as stated. Double quotes would be correct, or no quotes. Some string functions are didactic, indicating that they operate on two or more strings at once. Even that wouldn't fit, since dyadic functions have only two arguments, not two or more. Thus translate might be used to translate a value from the English character set to a Swedish or French character set.