“Dirty flashing” is a method of updating the Android operating system (OS) without wiping any data stored on the device. The term “dirty” refers to the fact that the existing data is not wiped before the update is installed, as opposed to a “clean” flash, which would erase all data before installing the new ROM. Dirty flashing is done for the convenience of the user, as it allows them to keep their personal files and settings intact after the update.
“Clean flashing” including Factory reset > Format data / factory reset via Custom Recovery is the safer way if a CustomROM is to be installed over another CustomROM, even if the code base is Open Source LineageOS, as is the case here.