Spreader What Do The Numbers On A Broadcast Spreader Mean ?

What do the numbers on a broadcast spreader mean ? - spreader

I planted some grass seed, which says it should be at 15 seeds per square inch, and 5 is too low and 30 is too high. My distribution reaches only 15, so I have a feeling that is not what the numbers mean that the package of grass seeds 15 proposes that an "intermediate" species would be the series. Ideas? Is a delimiter Kantenschutz Scott and instructions that came with it, do not really say what the numbers, unfortunately.