A Shortfill is a bottle containing vape juice
that isn't filled to the top, leaving space for optional nicotine liquid. It is filled just short of the maximum volume, hence its name 'Shortfill'.
When purchasing a short fill, you will note that it comes only in zero nicotine. This is because all nicotine-containing liquids are limited by law to a maximum capacity of 10ml whilst short-fills generally come in volumes greater than 10ml (such as 30ml, 60ml and 120ml bottles).
Each of the above example bottles sizes leaves either a 10ml or 20ml space in the bottle to make room for either one or two nicotine shots.
Generally speaking, by adding one 18mg or 20mg flavourless nicotine shot to a 50ml juice (that comes in a 60ml bottle) you will then create 60ml of juice that is approximately 3mg in strength. This is the most common nicotine concentration across vapers in the UK.
By adding one nicotine shot to a 100ml juice, you will end up with 110ml of juice at 1.5mg concentration. If you then add a second shot to that same juice, you will fill up the 120ml bottle and end up with 120ml of juice at a concentration of 3mg.