This key is a sort of magic key, i.e. one that isn’t actually read from the database. When you “read” it, it triggers the same mechanism on the client that captures status in fdbcli. At a high level, this involves talking to the cluster controller, who then does a variety of status collection and includes some data obtained by communicating with other processes in the cluster. It returns the result, the client adds a few things to it, and then the document gets returned to you.
It looks like the main other two are documented here:
There are also a couple that can be used for the purposes of killing processes in the cluster, but they are a little bit more complicated and I’ll defer the description of those to a documentation task.