Certain emojis or domain names can include a variation selector: U+FE0F or U+FE0E
These variation selectors are used by your browser to know whether the emojis should be displayed as texts or images. However, some emojis are displayed the same as text and image.
A list of emojis with their variations can be found here
This can lead to people owning different domain names that read the same (e.g 😀 with and without a variation selector)
A warning sign is displayed for all the domain names including a variation selector to make sure the variation selector is relevant for this emoji.