Suppose you have a method that returns a string. You want to use this in a 4DIF tag to see if the string returns "". You might be inclined to write this in your HTML:
<!--4DIF (SES_SessionUser="")--> : 4DENDIF expected
Even if SES_SessionUser is a valid method that returns a string, you will be presented with an error like this:
: A Boolean argument was expected.
Even though this is a Boolean argument, 4D needs a little help understanding what you mean. This will work instead:
Note the use of parentheses to signal to 4D that SES_SessionUser is a method.