Canon MARK IV User Manual (en)

 Protecting Images
If you format the card (p.50), the protected images will also be erased.
To protect an image, press and quickly let go of the <H> button. If you 
hold down the button for approx. 2 sec., a voice memo will be recorded.
Images can also be protected individually when the [3 Protect images
menu option is set to [Select images]. Press <0> to protect or 
unprotect the image.
Once an image is protected, it cannot be erased by the camera’s erase 
function. To erase a protected image, you must first cancel the protection.
If you erase all the images (p.179), only the protected images will 
remain. This is convenient when you want to erase unnecessary images 
all at once.
If [8C.Fn IV -9: H button function] is set to [1: Record memo 
(Protect: Disabled)
] or [2: Play memo (hold: Record memo)], images 
cannot be protected with the <H> button. Use the [3 Protect 
] menu option’s [Select images] to protect images.