I don't understand what is the question.
Let me explain:
/* line 1 */ #define INT int
/* line 2 */ INT a;
In the above code, if we disable "storing macro usage", we have two tokens in the token tree:
1, macro definition token "#define INT int" in line 1
2, global variable token "a" in line 2
But currently enable "storing macro usage" will have another token in the tree:
3, macro usage token "INT" in line 2
I just means that such macro usage may happens too many times in source code(they just like functions calls), and we don't need to store them in the token tree to save memory usages.