Clays are solid, so how can there be liquid clay? Well, that’s just because it’s the liquid form of polymer clay. It’s the same exact thing, but it doesn’t contain the thickener that makes it become solid, regular polymer clay. It’s the same otherwise.
That being said, each brand of liquid clay has a slightly different look, feel, and behavior. You can nearly always use them interchangeably for most situations where you’ll use liquid clay. Feel free to mix and match the brands you have on hand.