Параметр Range (Enum), как указано в документации -
Exclusive : The range does not include the to and from values.
Inclusive : The range includes the to and from values.
Использование в качестве ответа показывает
It.IsInRange(0, 100, Range.Exclusive); // will not match for int values 0 and 100, but matches 1 to 99
и
It.IsInRange(0, 100, Range.Inclusive); // will match for int values 0 and 100, and in-between