Short fills are bottles of e-liquid that contain more than 10ml of e-liquid. They are so called because they are filled short of the top of the bottle to accommodate the addition of nicotine shots if needed by the user.
As per UK law, bottles of e-liquid that are larger than 10ml cannot contain nicotine. This means that the user must add their own nicotine in the form of nicotine shots.
As an example, a 50ml short fill will contain 50ml of nicotine free e-liquid in a 60ml bottle. This can be used as is if the user does not need nicotine, but there is room to add one 10 ml nicotine shot to the bottle. This would make 60ml of 3 mg/ml e-liquid.
Short fills are ideal if you do not need a high nicotine strength and want to be able to carry plenty of e-liquid with you without having to bring multiple little bottles.
It is important to bear in mind that short fills are designed to accommodate enough nicotine shots to produce a 3 mg/ml e-liquid, so if you need a stronger nicotine strength, short fills may not be the right option for you.