Nicotine occurs naturally in tobacco leaves in the form of a salt. In this natural form, the active nicotine is not easily "freed up" so it can't be delivered to our bodies very effectively.
In the 1960s, cigarette companies developed a process which added ammonia to the natural nicotine salts from tobacco leaves which made the nicotine more bio-available and provided more of a nicotine hit without increasing the dose. The process they pioneered gave us freebase nicotine, which is the type of nicotine most commonly used in e-liquids. It is also used in nicotine replacement therapies such as patches, gums, lozenges and sprays.
Due to its potency, freebase nicotine provides a harsh throat hit at higher strengths. When it’s used in a high-wattage setups, it tends to be very uncomfortable on the throat, which is why vapers using sub-ohm setups tend to use much lower nicotine strengths. This low concentration prevents the e-liquid feeling too harsh on the throat when inhaled but these low strengths mean vapers have to vape for much longer durations to be satisfied.
Recently, a new chemical process was developed by the vapour industry to enable natural nicotine salt to be more bio-available and to more closely mimic the sensation of smoking a real cigarette. It utilises benzoic acid which helps nicotine salt vapourise at lower temperatures and makes it more absorbable like freebase nicotine. It also delivers the nicotine with a much smoother throat hit.
This new development allows users to obtain a nice potent hit of nicotine in a shorter time using a more portable low wattage device without the harsh throat hit.
Today's pod systems are ideal for salt based nicotine as are most of the standard higher resistance (1.5ohm plus) MTL tanks on the market. Keep in mind though, you should avoid using high powered devices because using sub-ohm devices to vape salt nicotine at high strengths will give you excessive amounts of nicotine which could cause unpleasant side-effects.