On my cheap "dollar store" scientific calculator, it has a 2nd function button named "CPLX". When you press it, the calculator displays some text similar to the "DEG, RAD, GRAD" that says "CPLX". When in this mode, you can't add, subtract, multiply, or divide. The equals button doesn't even work.
What is this and how is it used? I'm wondering if maybe it's for complex numbers, seeing that there's also a "a" and a "b" button next to it.