"BX", as in type AC cable, is a thing of the past. The new industry standard is type MC cable since the 80s. True, people still use the misnomer "BX". National codes require metallic wiring systems in structures over three stories, with some residential applications being exempt from this requirement.
By the way, I hear that Southwire and several other manufacturers are bringing back "BX" cable, but it now includes a #10 aluminum bonding conductor instead of the original flimsy aluminum strip.
As others have mentioned, never, ever, assume that an installation requires plenum rated cable. This can make or break a deal by thousands of dollars in material cost. Plenum cable isn't better, and non-plenum cable isn't inferior. It's all about what is required for the application.
