Shortfill e-liquids are basically e-liquids without nicotine that are optimized for usage with nicotine boosters (or Nic Shots) to create the nicotine level you desire. Shortfills come in different sizes designed for different users.
A 25-in30ml Shortfill usually uses half a 10ml nicotine boost 20mg to get a total of ±3mg of nicotine in the mixture, if you use a full 10ml nicotine booster you will create ±6mg of nicotine. With a 50-in-60ml Shortfill you can double the amount (10ml for ±3mg and 20ml for ±6mg) to create the same effect and with a 100-in-120ml Shortfill you can double the amount again (20ml for ±3mg and 40ml for ±6mg) for a similar effect. If you vape on ±6mg or higher it is possible you will need to put the e-liquid in a bigger bottle to fit the nicotine booster with your mixture.