My understanding is this. A dimensionless unit is a unit that points to real data but is itself empty. A ballistic coefficient describes a relationship of drag to shape and density. Drag is measurable, shape is, density is.
The bc is not inherently measurable. It’s an output and shorthand for actual measures.
For further clarity, take two objects with identical bc. One gets that number by being really aerodynamic. One gets that number by being really high in cross sectional density. Their shapes are different and densities ar different but end up with the same bc.
You can see how bc is dimensionless, it doesn’t tell us real world dimensions of an object but it’s more like a number rating system.
so if you read dimensionless again make a logic chart, does this describe a real measurement or is it empty of them and pointing to a possible set of real world measure