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Dirty flash is when you install an update without formatting or wiping the data partition. Dirty flash therefore means no loss of files if you do it correctly.
Clean flash is when you format the data partition. This therefore means loss of files unless you have backed them up to an external device like a computer or otg storage device
There is only a few times you may need to clean flash.
-When going from one major version to another like android 13 to 14.
-When downgrading.
-When your device maintainer says it's required because of changes.
-Maybe other times
Obviously we can't account for every user configuration...so backups are still advised when dirty flashing (Your device, your files, your responsibility). If something doesn't work after you dirty flash, then a clean install following your maintainers instructions and files normally gives you the same experience as everyone else who follows those steps.
Reading in your device specific support group will give you the most accurate answer for your specific device.