Brown Malt goes back further than Amber Malt in the history of English porter — it's the malt 18th-century London brewers used as the near-total base of the beer before pale base malt existed in its modern indirect-kilned form. Dry-kilned over wood fires to a deep brown colour, it carried enough of its own diastatic power at the time to convert an all-brown-malt mash, though the modern version, made with different barley and kilning practices, generally can't self-convert reliably the way period brown malt reportedly could.
At 45-65 °Lovibond, modern Brown Malt reads dry, biscuity, and mildly roasty, nuttier and less sweet than a crystal malt at a similar colour, and closer in spirit to Biscuit or Amber Malt's dry-kilned character than to chocolate malt's coffee-forward roast. It carries no meaningful diastatic power today and needs a proper base malt alongside it for conversion. Porter and old ale recreations aiming for genuine historical accuracy are its main modern audience, alongside some English milds wanting a dry, nutty depth. A blend of Amber Malt and a small amount of chocolate malt approximates Brown Malt's flavour reasonably well where the genuine product isn't stocked, though period-accuracy brewers generally consider that approximation a reasonable compromise at best, rather than a true substitute for the genuine, considerably harder-to-source original article itself, malted and kilned the old-fashioned way.