If the couple is not a married couple then the film is about exploring the shared lives people can create without knowing each other, how their simulation of a marriage is a direct approach to an aesthetic argument of simulation and replication in art. If the couple is married, then the film is about a long-married couple attempting to recapture the springtime of their marriage both by simulating their honeymoon and by replicating themselves as they were fifteen years ago, creating copies of past lives that no longer correspond to their current selves. Or perhaps the time frames are shifting, the question of whether they are married or not can't even begin to be answered, and the couple transforms by some mysterious pirouette from new acquaintances into a married couple. But if this is the case, of course, then the continuity of their day-long dialogue becomes a surprisingly streamlined thread between two disparate time frames, so that the film is at once continuous and discontinuous. Even if you wish to forgo any such interpretation, you are still left with many hallmarks of Kiarostami's Iranian cinema (a restrictive camera that refuses to show us essential information lying outside the frame, pairing of professional and non-professional performers, various methods of implicating the audience in the narrative, etc.) and countless other points of entry (such as recurring visual motifs, the emotional rawness of the performances, references and allusions to the European art film of the past, etc.).
But what primarily matters is the feeling of existing with these people in a bounded temporality, replete with ravishing visual textures and moments of pure cinematic beauty. Kiarostami not only exercises his uncanny ability to depict the changes in visual atmosphere that transpire over the course of a day, but he also synchronizes these changes to the characters' constantly shifting emotional climate. Most of the intellectual ideas of the film, as I have so rudimentarily outlined them above, take form later, after mulling the film over in your mind (believe me, I have far from exhausted all options or meanings; in fact, the more I think about the film, the more convinced I am that the possibilities are endless), but its human and emotional truths unfold moment to moment within the film itself, and once it is over it is almost impossible to recapture them.
In other words, I believe Certified Copy has surmounted Playtime as my favorite film.