I hate to sound stupid but I don't understand the numbers on the top of some of the pages in the pdf file. Like the 611, 612, 613 and so on. Could someone please explain this to me???? :confused: They represent the class of sirens by decibel rating. For example, 617 is the Thunderbolt class because...

I hate to sound stupid but I don't understand the numbers on the top of some of the pages in the pdf file. Like the 611, 612, 613 and so on. Could someone please explain this to me????