Constructing a complex number with an imaginary component equal to an integer zero simply returns the real number corresponding to the real component:

Constructing a complex number with an imaginary component equal to floating point zero will still output a new complex number, however:

Unlike math, where all real numbers are also complex numbers, Factor only considers a number to be a complex number if its imaginary part is non-zero. However, complex number operations are fully supported for real numbers; they are treated as having an imaginary part of zero.