this is how i understand it all..
Active tank: you are using sheild or armor repair units to continously repair yourself as you become damaged.
Buffer Tank: you increase the resistance lvl of your tank to EM, Therm, Explosive, and Kin. This will help you last longer in a fight because you will be taking less damage but unless you have someone runnning logisticts on you, you will eventually die unless you kill your opponets first.
Also with your buffer tank would be a passive sheild tank. This means that you have boosted the recharge rate of your sheild as high as possible so that hopefully it is repairing itself faster then the damage your taking.