It's a mathematically derived numeric value. It's derived by dividing the maximum possible target value by the target for the current block.

Here is the code of the math behind it from source code.

#include

#include

inline float fast_log(float val)

{

int * const exp_ptr = reinterpret_cast (&val);

int x = *exp_ptr;

const int log_2 = ((x >> 23) & 255) - 128;

x &= ~(255 << 23);

x += 127 << 23;

*exp_ptr = x;

val = ((-1.0f/3) * val + 2) * val - 2.0f/3;

return ((val + log_2) * 0.69314718f);

}

float difficulty(unsigned int bits)

{

static double max_body = fast_log(0x00ffff), scaland = fast_log(256);

return exp(max_body - fast_log(bits & 0x00ffffff) + scaland * (0x1d - ((bits & 0xff000000) >> 24)));

}

int main()

{

std::cout << difficulty(0x1b0404cb) << std::endl;

return 0;

}