It's just aesthetic, the top of the fork crown is shaped to blend into the enlarged head tube with integrated, but you can use an integrated fork with a conventional headset or vice versa. The exception is a fork designed with an integrated crown race, e.g. Look HSC5 and up, which must be used with the matching bearing. Some, but by no means all, conventional headsets use the same bearing cartridges, so even this is not a total bar to use in a conventional frame.
It's just aesthetic, the top of the fork crown is shaped to blend into the enlarged head tube with integrated, but you can use an integrated fork with a conventional headset or vice versa. The exception is a fork designed with an integrated crown race, e.g. Look HSC5 and up, which must be used with the matching bearing. Some, but by no means all, conventional headsets use the same bearing cartridges, so even this is not a total bar to use in a conventional frame.