Nicotine Salts e-liquids are complex compounds containing salt-based nicotine, typically less harsh to vape which allows for better tolerance of higher nic salt e-juice. Keep in-mind that nic-salt eJuice is typically different than traditional Free-Based Nicotine, the conventional eLiquid 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 that allows for a smoother vaping experience at higher nicotine strength -- optimized for lower-power devices such as pod systems and certain vape pens.
However, nic-salts with lower concentrations (less than 20 mg) of nicotine can also be used in sub-ohm devices, albeit with the caveat that they may reduce coil life. Why would you want to do this? Nic-salts can impart your favorite flavors or blends with a characteristic mouthfeel and more subtle flavor profile, so for some vapors, it makes sense to use them in their sub-ohm devices.
If that doesn't sound like you, play it safe and stick to using nic-salts, regardless of strength, only in lower-power devices like those from Aspire, Bo by J. Well, Innokin, Joyetech, Suorin, SMOK, and others.