For me, it's like a second sense of vision, which does not get hallucinated into the primary sense of vision and which cannot be confused with actual vision, yet nonetheless feels much like vision, with the exception of "not feeling real". By default it is "someplace else", but I can overlay it onto my primary vision if I want to (e.g. imagining an object on my real-life desk), but that doesn't affect my primary vision, merely gives me a sense that the imagined sight corresponds spatially with the actual reality in the intended manner.