The section starts with
A constant symbol starts with a digit (0-9) or a period.
You cannot change the value of a constant symbol. It is simply the string consisting of the characters of the symbol (that is, with any lowercase alphabetic characters translated to uppercase).
This is false, because environment symbols a) have a value that can be changed, and therefore b) their value is not their own name translated to uppercase. Indeed, it contradicts the next paragraph (after the example):
Constant symbols, where the first character is a period, which have at least one other character, and which are not a valid Rexx number, are environment symbols and may have a value other than the symbol name.
It is impossible that the value of a constant symbol cannot be changed and can be changed at the same time. Incidentally, the comma after "symbols" is incorrect, as it completely changes the sense of the whole paragraph (all constant symbols that X, Y and Z are environment symbols --> right; constant symbols, which are X, Y and Z, are environment symbols --> wrong).
Please note that 1.10.4.4 has it right: it says "Symbols that begin with a number or a period are constant symbols and cannot directly be assigned a value" (emphasis mine).
Anonymous