The current convention is all caps with underscores, italicized. The italics help a little, but nonetheless it looks to some (myself certainly) like either a constant or an environment variable. That collision is undesirable.

There are two well established conventions for replaceables out in the world. We should use one of them.

1. Uppercase with hyphens: BROKER-INSTANCE-DIR.
2. Lowercase with hyphens and brackets: <broker-instance-dir>.

I personally think the latter is a good choice, particularly because it does a good job of visually distinguishing a part that is replaceable from a part that is not, as with "<install-dir>/bin/artemis". But any approach that created sufficient distinction will do. It could be boldface when it's part of another value, for instance.