These are for split grade printing. So for example, you put in the number 2 filter and make your test print or strip to gauge the exposure time. See number 2 as neutral in light or dark contrast. You don't have to do split grade printing and if you have a good negative you can go ahead and print your picture on a grade 2 or any other filter. As the numbers to up so the darker the contrast becomes.

For example, here is a basic scenario.

>You make a test strip on Grade 2 and discover your optimal exposure time is 20 seconds.
>You find that all tones look good and that grade 2 is sufficient so you go ahead and print on grade 2
>You don't have to use split grade printing on Variable Grade paper, so you could decide on more contrast and use a Grade 3 filter (or vice versa)

>You make a test strip on Grade 2 and discover your optimal exposure time is 20 seconds.
>You split the exposure into 2 part of 10 seconds
>10 seconds with a grade 5 filter and 10 seconds with a grade 0 filter
>You can use any grade up and down the grade scale depending on what you want

Split grade comes in handy when doing lots of dodging and burning and controlling area specific contrast.

I find I need to be careful about split grade (as a newbie in printing) and first check what I can achieve with a good negative and a straight up single grade exposure.

Split grade is fun though.

So, use neutral grade 2 to get your exposure time on a test strip, then divide that time up into however much of the two multigrade paper emulsions you want to expose. For a simple exposure, split in half and try the two extremes (grade 5 and grade 0) then decide what would be optimal from there.

There is no standard, use and do what gives you the effect you want.