There are different size adapter brackets that mount the caliper to the fork/frame. They are usually sold by mount type and disc size.
Mount type is fairly straight forward. PM or IS. Aka Post Mount or International Standard.
Disc size being the diameter in mm. The front and rear both use the same brackets, but due to the location the front and rear disc size for those mounts is different. Some are listed universal, and will state the size for the location (180 front/203 rear) Others are just sold for one location, but can still be used for both.
This is the brand I bought most of my brakes through for my current bike. Their listing actually shows a lot of this detail.
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