Relative to the electrical wire rating of nm-b, I understand the nm stands for non metallic but what does the b designate?

The letters NMB on the jacket of the wire. The NM means nonmetallic sheath cable (Romex) and the B suffix means that the cable's conductors are rated for a maximum operating temperature of 90°C 194°F. We can assume that wiring made prior to 1984, without the B suffix, is rated at 60°C 140°F. This is the type of wiring found in most older homes that were built before 1982. This is one source.

Thanks Stratmando!
Best regards!
TG