This folder serves as a common repository for application-specific data for current user in Windows operating system. A typical path used by BitComet is C:\Documents and Settings\%userprofile%\Application Data\BitComet (Windows XP) or C:\Users\%userprofile%\AppData\Roaming\BitComet (Windows Vista or later). (This folder is being used starting with v1.18)
A typical installation path of BitComet is C:\Program Files\BitComet.
Saves data of pieces where the file ends within a transferred piece, if the user is only downloading some, but not all of the files in a torrent. Refer to FAQ for more info.
The APP_DATA folder is used by BitComet since v1.18. A new option “system.use_app_data” has been added to the Advanced page of the Options dialog. Users can switch the configuration folder of BitComet using this option.
For a new installation
If installed in PROGRAM_FILES folder, the default configuration folder will be in APP_DATA; otherwise the default configuration folder will be the installation folder. The latter situation is useful when installing BitComet in a removable USB disk.
For an upgrade
BitComet will seek and find the existing configuration files in their current installation1) folder, and their installation folder will continue being used as the configuration folder, no matter if it's the PROGRAM_FILES folder or not.
For multiple installations in a computer
BitComet will try to seek and use the configuration files in the current installation folder at first. If not found, it'll try to search for them in the APP_DATA folder. If still not found, it'll create new configuration files as in a new installation, in the APP_DATA folder or installation folder.
How to downgrade
Check the option “system.use_app_data” in the Advanced page of the Options dialog. If it's set on true, disable it (set it to false) to have all configuration files moved back to the installation folder. After that, reinstall the old version of BitComet, and it should work properly.