So whats the deal with the intake numbers? What does the number below the TB mean if anything?

One thing that I am noticing that is probably normal. It seems to surge at idle when warmed up and under part throttle. Its probably due to the tune or lack thereof, right? But I can hit a spot at about 1600rpm where it will chug a little. Its not an opti chug because its responsive as all hell. Even though it chugs a little I can still floor it and it takes off like a bat outta hell. Any ideas?