Shortfill e-liquids derive their name from the fact that they are bottles containing juice that have not been filled to the very top. These refills are not nicotine containing e-liquids and as such, the liquid that comes in shortfills is always zero nicotine. This is because it is prohibited to sell any nicotine containing liquids in bottles over 10ml. As such, shortfills usually come in either 60ml or 120ml bottles that have been filled to either 50ml or 100ml respectively. This means that in a 50ml shortfill (that is contained in a 60ml bottle) you will have a 10ml gap when you receive your bottle. This gap can then be filled with a nicotine shot. By adding one 18mg or 20mg nicotine shot to a 50ml zero nicotine shortfill, you will achieve an overall concentration of 3mg in the newly mixed 60ml e-liquid.