Thread:BunsenH/@comment-25404524-20160424062328/@comment-24577221-20160424222704

If that welcome message contains any of those four items in its text, when the message is sent to a new person, those items are replaced by the items they represent. Take a look at http://mysingingmonsters.wikia.com/wiki/MediaWiki:Welcome-message-wall-user. It uses $1, $2, and $3. The first line includes "Welcome and thank you for your edit to $1!" Now look at the message that has been posted on someone's wall -- the '$1' is replaced by the page that that person just edited.

I don't think that particular set of substitutions are used for any other pages or messages. It's somewhat like a template that's built into the wiki structure; it recognizes those particular variables and makes those substitutions.