The question is regarding charindex function!!

DECLARE @document varchar(64)

SELECT @document = 'Reflectors are vital safety' +
' components of your bicycle.'
SELECT CHARINDEX('vital', @document, 5)
GO

In the above statement what does 5 do for me!!
i read it is the starting point of the search!! but it does not make any difference whether i have 5 there or not!!
The result will be 16 with 5 or without 5!!
Pls help!!
Thanks!!!