A Series - most features, JetPak system, upgraded lighting, touchscreen control, etc
R Series - JetPak system but not quite all the bells & whistles of the A Series, no exterior lights, no touchscreen option
X Series - Bullfrog's value line, NO JetPaks, built similar to the rest of the spas on the market but with a maintenance free build just like the A & R Series
Each Series has all of the models...think: Good, Better, Best...so if you like the 7'4" x 7'4" x 36" size it is available in the X7, R7, or A7....same exact size/interior layout but at 3 different price points and also 3 different levels of 'features'...each corresponding model also has a lounger version so again X7L, R7L, A7L of course the L simply denotes lounger.
Hope that Helps! Bullfrog makes a great spa