The term sub-ohm (sub-ohming and sub-ohm vaping) used to refer to having a coil resistance under 1 ohm. Vaping over 1 ohm is more for flavor and MTL style. Vaping with lower resistance for direct lung vaping (clouds!).
However, the term 'Sub-Ohm Tank' is now understood to mean a tank that has a pre-built coil insert that has a low resistance, regardless of the type of wire (kanthal, titanium or otherwise).
When you are looking for a good sub-ohm tank, you pretty much want an easy to use, pre-built coil that can really bring huge cloud production.