Fines in a concrete mix
Concrete aggregate is graded from coarse stone down through sand, and the fine end of that spread is what gives the batch its paste and its workability along with the cement. A mix that is short on fines is harsh, hard to finish, and hard to move. A load carrying a full spread of sizes, coarse down through fine with nothing missing in the middle, travels and finishes better than one with a gap in that gradation.
In this context, fines are a wanted part of the recipe, sized and proportioned on purpose by the plant against the mix design.
Fines in base stone are a different story
Under a slab, crusher run and similar base stone need some fine material too, because the dust and small particles are what let the stone lock tight when it is compacted. Too little, and the base stays loose and shifts. Too much, especially clay and silt washed in from the ground underneath rather than in the stone itself, and the base holds water instead of draining it, which is the opposite of what a base is for.
That second kind of fines is a soil condition more than a material spec, and it is exactly what a crew is checking for when it looks at what an excavation turns up before the base goes down.
Why the difference matters on your job
The fines in your concrete order are the plant's problem, chosen against the strength and slump you specified. The fines under your slab are a site condition, and it is the reason a crew looks at what the excavation turns up rather than assuming every lot compacts the same.
Around here that usually means red clay, which holds water and does not drain the way a clean crushed base does. Where that shows up, it gets handled before the concrete goes in, not discovered after.
That is also why a base gets proof rolled or otherwise checked before stone goes down at all. A subgrade that looks firm and packs soft under a load is telling you something about its fines that a look from the surface will not.