Interestingly, it seems that different localities tend to use different jargon for stone sizing, with not much standardization. There is no government agency that regulates it. :)
Regardless, since the sources of stone tend to use sieves with defined size holes to sort the stone, this tends to tell you the size of the stone to use. So around here, #3 stone means that which will fit through a sieve with 3 inch holes. In some cases, the stone will already have been run past smaller sieves.
I will suggest that stone with dirt in it will tend to support the growth of weeds, encourage tree roots more, etc. And since these stones tend to be more round, with few sharp edges, they don't really pack that well for a driveway base.
I have also seen that "crusher run" tends to pack very solidly. This is crushed limestone, run through the same sieves to size the stone. The sharp edges of crushed stone prevent things from sliding around, and the presence of smaller crushed stone fills in the interstices, locking things together even more. In fact, #3 crusher run seems to be a common base for driveways around here, with smaller sized crusher run on top as a surface. Our gravel driveway is made of exactly that.
So I would not be at all surprised if the heavy equipment guy was in fact referring to #3 crushed limestone for a base.
If this is an interior installation then no powder/gravel based playing surface will be "dust free" as the impact of the balls will create dust. Reading up on the game, it does say that it can be played on grass.
Based on that, for an indoor game, I'd go with hard rubberized flooring (such as used in gymnasiums) topped with interior AstroTurf of a short cut.
Best Answer
Generally anything that is "modified" means it has some limestone dust mixed with it, the idea is that once it is compacted, over time as rainwater falls on it, the water works down through it and causes it to harden into a unified piece of rock like concrete. Sometimes they call it "treated" instead of "modified" The "minus" stuff is just whatever comes out of the crusher. For example 3/4 minus is gravel that has rocks no larger than 3/4" but has rocks that can go down to the size of sand grains.
As for what goes into it, "minus" can be anything from broken chunks of concrete, to actual rocks, to a mix of bricks, rocks and concrete all fed into a crusher and sifted to keep rocks out of it that are larger than the rating.
The minus stuff is generally what you put down when you are planning on putting asphalt or concrete on top of it.
The modified is what you put down when you aren't going to cover it but want a surface that will eventually firm up so you can park cars on it, etc. It can take a year for that to happen, though.