Amber Malt predates the modern split between "base malt" and "specialty malt" — in 18th-century English brewing, before precise, indirect kilning made a genuinely pale base malt widely achievable, beers now recognised as early porter were built largely or entirely on malt kilned to an amber colour, since that was close to the practical limit of what kilning technology of the era could reliably produce pale.
Modern Amber Malt is a specialty product reviving that character rather than a base malt in its own right — dry-kilned like Biscuit or Victory Malt rather than stewed like a crystal malt, giving it a biscuity, mildly toffee-ish, nutty flavour with no diastatic power of its own to speak of. At 20-30 °Lovibond, it reads a solid amber-brown, and its conventional ceiling around 15% reflects a role as a substantial flavour contributor rather than a token accent. Historical porter and old ale recreations reach for it specifically to approximate the pre-black-patent-malt brewing tradition — the same malt-tax-driven history that produced Black Patent Malt in 1817 also marks the point where brewers largely moved away from all-amber grists toward pale base malt plus a small proportion of heavily roasted malt instead. Biscuit Malt or Victory Malt are the closest modern substitutes where Amber Malt specifically isn't stocked, though neither carries quite the same specific historical association with the earliest porter recipes.