It seems that the two terms concealed and implied are used as synonyms when it comes to nude photography.
They are not synonyms; in fact, they mean something entirely different.

Concealed Nudes

The term Concealed Nudes describes an image where the model is nude, and the primary and secondary sexual characteristics are hidden. So no bits and pieces are on show, but, the model is obviously nude.

Model: Angel White

Implied Nudes

The term Implied Nudes describes an image where the model might be nude but not necessarily has to be. The framing implies it, but the model might wear clothes. A good example is a portrait with bare shoulders; the viewer does not know if the model is nude or dressed.

Model: Jezebelle

It is no so hard to use the right terms once you know the difference.