I don't know the exact details, but the 6-port intake was introduced on the 13b motor in the 1984 RX-7 GSL-SE.

This is a 4-port manifold:



This is a 6-port manifold (disregard the carbeurator sitting next to it):



There's a valve somewhere that causes the secondary ports to kick on, I think it might have something to do with exhaust backpressure but I forget exactly how the system works.

3-stage I would presume means that in addition to the 6-port system, the intake manifold has some kind of variability to its length kind of like the GSR manifold.