Canon EOS 5DS Instruction Manual page 89

AF Operation Indicator
In One-Shot AF mode, the icon also appears if you press the shutter
button halfway after focus is achieved.
AF Points Lighting Up in Red
By default, the AF points light up in red in low-light conditions. In the
<d>, <s>, <f>, <a>, or <F> mode, you can set whether to have
the AF points light up in red (p.132).
f: Selecting the AF OperationN
While you press the shutter button
halfway and the camera focuses with AF,
the <e> icon will appear under the
focus indicator in the viewfinder.
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