Salt Nic E-Liquids are complex compounds. These are containing salt-based nicotine. Thus, these are typically less harsh to vape. Which allows for better tolerance of higher salt nic e-juice.
Keep in mind that Nicotine Salt e-juice is typically different than traditional Free-Based Nicotine, the conventional e-liquid you see in bigger vape devices.
For the most part, both types consists of the same nicotine found within the natural tobacco leaf, with the exception of benzoic acid composition in nicotine salts.
As a shortcut guide, benzoic acid lowers the pH level and reduces alkalinity. This allows for a smoother vaping experience at higher nicotine strength. Optimized for lower-power devices such as pod systems and certain vape pens.