Interesting question. I'd like to say unfired, since I would surmise that it's the raw clay that supplies the nutrients to the plants, but raw clay would dissolve. Unless it's mixed with something else or it's a "different" type of clay -- ie: not clay as we know it.
I use Flourite, myself, and it is also clay based. From the SeaChem site:
"Flourite™ is a specially fracted stable porous clay gravel for the natural planted aquarium. . . . Flourite™ is not chemically coated or treated and will not alter the pH of the water."
Fracted, stable -- Hrm, not chemically coated or treated. Sounds like fired clay to me. FWIK the only way to stabilize clay is to fire it.
Heh, here I have this vision of mad scientists throwing terra cotta pots on the ground and screaming, "Eureka!"
I think, unless someone else has already dug into it and has the answer, the only way to really know is to email the makers.
But if I were to hazard a guess on Flourite and laterite, I'd say it's smashed up or powdered fired clay mixed with potting soil, or the clay is fired with potting soil and/or nutrients mixed in and then smashed up or powdered.
Roan