When a character in the image seems to be a futanari, but there is no visible penis.
Whether the character is canonically a futanari or not, whether related pictures have a penis showing or not, and whether the artist clarifies that it is futanari or not aren't relevant. As long as it looks like futanari but you can't see a penis or evidence of a strap-on in the picture, it is implied futanari.
This tag and the futanari tag are not mutually exclusive.