Canon EOS M6 Mark II Advanced User's Manual page 146

ISO Speed Settings
ISO Speed Guide
Low ISO speeds reduce image noise but may increase the risk of
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camera/subject shake or reduce the area in focus (shallower depth of
field), in some shooting conditions.
High ISO speeds enable low-light shooting, a larger area in focus (deeper
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depth of field), and longer flash range but may increase image noise.
Image noise (such as dots of light or banding) may increase and apparent
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resolution may decrease at H (equivalent to ISO 51200), because this is an
expanded ISO speed.
If [z: Highlight tone priority] is set to [Enable] or [Enhanced] ( = 152),
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ISO 100/125/160 and H (equivalent to ISO 51200) cannot be selected.
When shooting with a high ISO speed, high temperature, or long exposure,
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image noise (rough grain, dots of light, banding, etc.), irregular colors, or color
shift may become noticeable.
When shooting in conditions that produce an extreme amount of noise, such as
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a combination of high ISO speed, high temperature, and long exposure, images
may not be recorded properly.
If you use a high ISO speed and flash to shoot a close subject, overexposure
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may result.
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