What is the 'Matrix User' option?

Unfortunately, no one can be told what the Matrix User is. They have to see it for themselves.

Matrix is a decentralised protocol for messaging and calls and video, it's like discord, but encrypted and decentralised. Element is the more popular client bacause it implement almost everything. and it's pretty good, although pretty buggy with calls, but they're working on it!

Place for your account on matrix, which you can to with Element for example to chat with your friends and groups.

It's working well but it's not very popular, unfortunately.

if you have a matrix user, you can put it in the field to allow other people to message you