I've been working on a project to update some of our Community Specific closure reasons on Stack Overflow.

In my research, I've been looking network wide at our various community's closure reasons. I've noticed the usage of what appear to be replacement strings in the markdown for the community specific reasons. 

For example, here on MSE this close reason: 
> This question does not appear to be about the software that powers the Stack Exchange network, within the scope defined in the [help center](/help)

appears to be written with the following markdown ([SEDE MSE query](https://data.stackexchange.com/meta.stackexchange/query/1594040))
> This question does not appear to be about $Topic, within the scope defined in the [help].

I know that \[help\] becomes [help center](/help) because of the [comment formatting rules](https://meta.stackexchange.com/editing-help#comment-formatting). I was wondering about `$Topic` which becomes "the software that powers the Stack Exchange network".

I knew of $SiteName and that it is used in many help center documents and tours. Stack Overflow uses it in several closure reasons ([SEDE SO query](https://data.stackexchange.com/stackoverflow/query/1573733)). I also know you can [search the help center with $SiteName](https://meta.stackexchange.com/help/search?q=%24SiteName) and it will give you results that match the current site's name. But I was unaware that there were more available.

Is a complete list of these replacement strings documented somewhere? Is there a way to know what the values of these replacement strings will be for a given site?