Because gnu.org says so: “The exponent in a hexadecimal floating constant is an optionally signed decimal integer that specifies a power of 2 (not 10 or 16) to multiply into the number.” https://www.gnu.org/software/c-intro-and-ref/manual/html_node/Floating-Constants.html

From the documentation (emphasis mine):

If the floating literal begins with the character sequence 0x or 0X, the floating literal is a hexadecimal floating literal. Otherwise, it is a decimal floating literal.

For a hexadecimal floating literal, the significand is interpreted as a hexadecimal rational number, and the digit-sequence of the exponent is interpreted as the (decimal) integer power of 2 by which the significand has to be scaled.

double d = 0x1.4p3; // hex fraction 1.4 (decimal 1.25) scaled by 2^3, that is 10.0

You can find the full documentation here: cppreference.com

So in your example 0x1P1 means 1₁₆ * 2^(1₁₀) = 2

I’ve never seen P used as an operator and find nothing with a quick Google search. What’s it do?

Edit: What’s it supposed to do/where did it come from?

1P1 = 1 * 2^1 = 2

Hex digits are 0-9 and A-F. I don’t think P is a digit.