Implied nude is where the model generally is fully or partly nude, look or appears to be nude, but where neither nipples or genetalia (slit, bush, pussy or whetever you want to call it) is showing. The edge of a nipple may be acceptable.
It could be that the front genetalia is in plain view but in deep shadow, the body could be turned to obscure, or cloth/underwear or props could cover. Generally the buttocks (ass more bluntly put) can be visible but normally photographed a bit off to one side or from above. This type of images are quite common in "nicer" mens magazines such as Euroman, GQ, S-Magazine, but also Cosmopolitan, Elle, Vogue etc.