Short fill e-liquids are 0mg e juices that are supplied in a bottle which leaves enough space for a nicotine shot. Nic shots usually enable you to make your vape juice up to 3mg nicotine strength.
Some examples of the different formats of short fills available are as follows:
60ml bottle containing 50ml of 0mg vape liquid. Add 1 x 18mg flavourless nic-shot to achieve 60ml of 3mg juice.
100ml bottle containing 80ml of 0mg vape liquid. Add 1.5 x 18mg flavourless nic-shot to achieve 100ml of 3mg juice.
120ml bottle containing 100ml of 0mg vape liquid. Add 2 x 18mg flavourless nic-shot to achieve 120ml of 3mg juice.
Once you have added your nic shot(s), you need to make sure the bottle is shaken vigorously (and make sure the cap is done up tightly first!).