High-bandwidth Digital Content Protection (HDCP) is a digital copy protection system that is commonly associated with DisplayPort, DVI or HDMI connections.
Cam Link 4K cannot receive any HDMI signal that has HDCP protection. It can receive HDMI signals that are not protected, like from many cameras.
If you connect a HDMI signal that has HDCP protection, then Cam Link 4K will not display video.
Some products may use HDCP when you don't realize it. For example, many Android devices may use HDCP often.
iOS devices only tend to use HDCP when you are watching video you have paid for in some way (like from iTunes or Netflix). iOS devices don't use HDCP during gameplay.