Answer 1

Let's consider one example :
You have to define a variable pie who's value is 3.14 now for that you write code
define ("pie",3.14,true);

Now if you declare it as true then to access this value you can use any variable (which is not case sensitive )that is either, "pie" or "PIE"
AND
if you declared the value as false then for above example you cannot use "PIE" to get value of 3.14