Canon EOS-1D X Mark III Advanced User's Manual page 777

4.
Add an item.
Press the < > button.
Turn the < > dial or use < > to select an item to add, then press
< >.
To remove an item, either select the item and press the < > button or
select [Clear all items] in step 2.
For items with a customizable display size, once more, turn the < >
dial or use < > to select a size, then press < >.
For details on the items available for the layout and their display sizes,
see Available items and display sizes for the screen layout.
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