All of this ties together with your other questions/threads...
If you were putting that B&C driver, the 18TBW100, into a Devastator that was tuned at 20Hz, you'll see a recomendation made for a 2nd order filter perhaps at 18Hz... This is just to protect the Driver from being pushed beyond it's "safe limits" which are already being pushed by the sheer fact that that Driver is in a box tuned so far below it's Fs. Doing so, even at more moderate output levels will still require significant power.
Some of the Devastator options I've seen they use a steeper slope because the driver modeled for that box it being pushed closer to it's margin.
To the best of my knowledge, for these HPFs, it doesn't matter whether it is a Butterworth or Linkwitz-Riley filter... that I've seen. I would love to see a better explanations of how to properly set an HPF (in terms of BW v. LR, and a more detailed look at the other parameters).