The SMOKTech Purex Dual Coil Tank (DCT) features a cartomizer design and a 3.5mL e-liquid capacity.
The Pyrex DCT has a glass tank wrapped in aluminum for increased durability. It has a capacity of 3.5ml, but is about the size of a 6ml tank due to its thickness. Like many of the Smok tanks, the coil is a cartomizer that slides through the center of the tank.
Glass tank atomizers are generally chosen to avoid tank damage while using highly acidic e-juices in electronic cigarettes (Citrus, cinnamon, banana, and heavy menthol flavors).
As an aside, the actual technical term for this device would be "SMOKTech Dual Coil Tank Electronic Cigarette Cartotank Clearomizer". There are gobs of terminology in the e-cig world; don't get confused! Once you get the hang of it, the naming of devices like this is actually pretty simple.