As you know, NISMO make a wide range of aftermarket parts and also complete cars. These parts and levels of tuning of cars are classified into 3 main categories.
1. S TUNE
2. R TUNE
3. Z TUNE
S Tune you could say is "Sports Tune". Although this may or may not be the correct abbreviation explanation, you get the idea. S Tune parts and cars are developed to be used in street applications mainly. Products such as mild upgrade brake pads, air filter element, suspension packages (shocks and springs as opposed to a full coil over setup) etc...
As such NISMO's S TUNE cars are also tuned for street use and are not tuned as hard as some circuit based cars are. For example the S TUNE engines you can buy for the GTR are not as powerful nor do they use as many parts as the R TUNE equivalent.
R TUNE is what you could call "Race Tune" where the parts are developed to be used on the circuit. As such products such as brake pads and suspension packages are more suited to circuit use. The pads have a higher heat rating and the suspension packages are a coil over setup as required for racing on the circuit.
R TUNE complete cars are also tuned harder to give better performance on the track rather than just putting around the street. As such they are also a little less practical on the road.
Z TUNE is rarely used for NISMO's Spare Parts. The main reason for the use of "Z TUNE" is for when NISMO built the latest incarnation of they're R34 GTR Complete Car, the NISMO R34 GTR Z TUNE. Details of that car can be found on the first link you posted. This car's capabilities and level of tuning are far beyond an R TUNE and is tuned to be the ultimate GTR ever offered for sale.
And just to throw a spanner into the works, NISMO also offer some complete cars as F-Sport GT's and S Tune GT's
I hope that clears it up a bit for you