Nicotine salts are a different form of nicotine to standard e-liquid, and it's why they feel so much smoother even at higher strengths. Rather than the harsher hit you'd get from freebase nicotine at the same mg, nic salts deliver quickly and cleanly, which is exactly why they work so well in pod kits.
Available in 5mg, 10mg and 20mg, they suit smokers switching over who want a satisfying hit without the harshness, as well as anyone moving on from disposables. Mostly blended at 50/50 for that familiar mouth to lung draw.