Why would I want to use this? "Since most RPGs are improvisational and we won't know what will happen till it happens, it's possible the game will go in a direction people don't want. An X-Card is a simple tool to fix problems as they arise... It's nearly impossible to know what will trigger someone."
By having an X-Card, you (as a dungeon master) will be able to craft a story that everyone can enjoy, and your players can have the assurance of knowing they can stop something they don't like.
Can I use this for more than just content issues? Sure! You could have a button that just asks for a break, even. (I do!) It's entirely up to you how to use the buttons you create.
How do I tag a specific user or a role in the message? First, you'll need the user's ID (or the role's ID). To get either of those, you'll need to enable Developer Mode. Click the "User Settings" cog icon at the bottom of Discord. Then, go to the "Appearance" section, and scroll to the bottom. Finally, enable the "Developer Mode" option.
If you want to tag a user (such as the DM), you can right-click on their name anywhere in the server and click the new "Copy ID" option. Then, in your message (on this website), type this code: <@1234>. Be sure to replace the "1234" with the User ID you copied!
If you want to tag a whole role, you'll need to go to the Server Settings section, followed by the "Roles" section. You can then right-click on the role you want to notify, and click the new "Copy ID" option. Then, in your message (on this website), type this code: <@&1234>. Be sure to replace the "1234" with the Role ID you copied!