The guru interface supports two representations of complex numbers, which we call the interleaved and the split format.
The interleaved format is the same one used by the basic and advanced interfaces, and it is documented in Complex numbers. In the interleaved format, you provide pointers to the real part of a complex number, and the imaginary part understood to be stored in the next memory location.
The split format allows separate pointers to the real and imaginary parts of a complex array.
Technically, the interleaved format is redundant, because you can always express an interleaved array in terms of a split array with appropriate pointers and strides. On the other hand, the interleaved format is simpler to use, and it is common in practice. Hence, FFTW supports it as a special case.