The label will say slow dry retarding "fluid" on it. That's the one I use in modeling and airbrushing. My label is grey but my bottle is also several years old. I don't know what color the fluid bottle might be these days.
As an experiment, one time I tried the retarding "medium" in airbrushing. It worked but took more thinning.. The fluid is just that, thin like water.
As far as I know, from Liquitex, there are only the two retarding products. Labels may have changed, along with size options. But yes, there are several other types of acrylic mediums for various purposes, from pouring medium to effects.