Canon EOS-1D X Mark III Advanced User's Manual page 398

Lens serial numbers
In step 3, if the asterisk "*" appears in front of the 10-digit lens
serial number, you can register only one unit of the same lens
model. Even if you enter the serial number, the asterisk "*" will
remain displayed.
The serial number on the lens may differ from the serial number
displayed on the screen in step 3. This is not a malfunction.
If the lens serial number includes letters, enter only the numbers.
If the lens serial number is eleven digits or longer, enter only the last
ten digits.
The location of the serial number varies depending on the lens.
Certain lenses may not have a serial number inscribed. To register a
lens that has no serial number inscribed, enter any serial number.
Note
If [Adjust by lens] is selected and an extender is used, the
adjustment will be registered for the lens and extender
combination.
If 40 lenses have already been registered, a message will appear.
After you select a lens whose registration is to be erased
(overwritten), you can register another lens.
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