Canon EOS-1D X Instruction Manual page 74

Changing Settings
Directory Structure of the Target Folder
Configured in [FTP server]  [Directory structure].
Selecting [Camera] automatically creates a folder structure matching
that of the camera's (such as A/DCIM/100EOS1D) in the server's root
folder for image storage. If you have created a subfolder in the root
folder by changing the [Target folder] setting, a folder structure such as
A/DCIM/100EOS1D is automatically created in that folder for image
storage.
Selecting [Default] will use the root folder for image storage. If you have
created a subfolder in the root folder by changing the [Target folder]
setting, images are saved in that folder.
Overwriting Files of the Same Name
Configured in [FTP server]  [Overwrite same file].
When the camera is configured to prevent overwriting
If there is already a file of the same name in the target folder on the FTP
server, a suffix consisting of an underscore and a number is appended
to the new file's name. For example: IMG_0003_1.JPG.
When you resend images if initial transfer fails
Even if the camera is configured to overwrite files of the same name, if
you resend an image file that could not be transferred initially, the
existing file may not be overwritten in some cases. If this happens, a
suffix consisting of an underscore, a letter, and a number is appended
to the new file's name. For example: IMG_0003_a1.JPG.
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