Everyone draws slightly differently, some hard and fast some slow and long all of these things play a part when using nicotine based vaping products.
Mouth to Lung is used to describe when the user takes the vapour into their mouth and then inhales it into the lungs. This is the most common way people vape or smoke
Direct to Lung is used to describe when the user takes the vapour straight into the lungs bypassing the mouth. This type of draw is used to create bigger vapour clouds.
Tanks are usually designed for one way or the other. The easiest way to tell is if they have large air channels they are for direct to lung and if they have small holes to let the air into the tank they are designed for mouth to lung.
The restriction of the air channels is what makes the mouth to lung vapes the most natural choice for people looking to switch from traditional smoking.
There are devices that cater for a best of both worlds this is known as Restricted Direct to Lung but still requires the vapour to be taken straight to lung rather than more natural way most people smoke which is Mouth to Lung.