Short fill vape juices are nicotine-free and come in underfilled ("short filled") bottles, leaving room for you to add your own nicotine.
These thicker E-Liquids are specifically designed for advanced vapers using sub-ohm vape kits, DTL tanks, and mods and are not compatible with MTL devices.
Shortfills came about when TPD was introduced, which limited nicotine-containing E-Liquid bottles to 10ml. Shortfills allow you to buy more E-Liquid at once, since they have no nicotine.
This is a cost-effective workaround for sub-ohm vapers, as their DTL vape kits tend to consume more E-Liquid per puff.
These e-juices are meant to be vaped at lower nicotine strengths of no more than 6mg. This is because higher strengths could be unpleasant on the throat with DTL vaping.