There's a difference between "freebase" nicotine, and the newer "nic salts".
Freebase nicotine is the purest form of nicotine and is the default "go-to" for most manufacturers. This solution of nicotine is simply added to a mixture of vegetable glycerin (VG) and propylene glycol (PG) which "carry" the nicotine with them when vaped. It's also what is used in nicotine replacement therapies—like gums, patches, sprays, and lozenges.
Nicotine salts are comprised of the same exact nicotine that's found in its natural state within the tobacco leaf. More suitable for the mouth to lung vapors. Nic salt is not to vaped at a high temperature or wattage. Best used in pod systems.