Canon EOS 6D Instruction Manual page 317

About the Lens Serial Number
In step 3, if " * " appears in front of the 10-digit lens serial
number, you cannot register several copies of the same lens
model. Even if you enter the serial number, " * " will remain
displayed.
The lens serial number on the lens may differ from the serial
number displayed on the screen in step 3. This is not a defect.
If the lens serial number includes letters, enter only the numbers in
step 3.
The location of the serial number differs depending on the lens.
Some lenses may not have a serial number inscribed. To register a
lens that has no serial number inscribed, enter any serial number in
step 3.
If [2: 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.
Enter the serial number.
Press the <U> key to select the
digit to be entered, then press <0>
to display <r>.
Press the <V> key to enter the
number, then press <0>.
After entering all the digits, select
[OK] and press <0>.
8: AF Microadjustment
317