They are telling you what size of lower headset you need in terms of the Standardized Headset Identification System (SHIS).
This is a naming system that tries to simplify headset standards in part by rounding as much as possible to the nearest whole number.
This frame's lower bearing interface is a very common size known under SHIS as IS52, otherwise known as 1.5" IS. The frame bore size is around 52.1mm and the bearing OD is exactly 52.00mm, hence the simplified name IS52. The /40 and the /30 refer to the crown race. If you were using a tapered fork, it's got a fork race seat OD of 39.8mm and a crown race ID of 39.7mm, simplified to just "40" under SHIS. If it were a straight fork, the fork race seat OD is 30.1mm and the race is 30mm, simplified to just "30".
What's confusing about what they've done here is that on their spec page, for the headset upper they only say "Integrated Campy Spec Upper Bearing (45x45 degree)." They probably should have also said IS42, which is the SHIS name for the same thing. As for the reason they're mixing standards like this, it's because the Campagnolo Hiddenset standard got an early foothold within the BMX/DJ world and is the standard to this day, and a lot of the headset uppers (key logo display points) people will want to use on frames like this use that standard, but they also wanted to make a frame that could take both tapered and straight forks, which wouldn't have been possible with the Campy standard. So you need a more BMX-y upper. If you have trouble finding just the upper, buying a whole headset isn't too bad of a waste of money because you're mostly just paying for an extra bearing, which you use as a spare when the first one wears out.