iPhone 7 Plus Portrait Mode is at heart an creative effect, so we should possibly be saying that there are no rules, experiment and be creative etc. But there are some rules, really.
You need the subject of the photo to be fairly close – the feature itself recommends 2.5 metres or less. The background, by contrast, has to be expressively further away: larger the gap between subject and background, the more pronounced the depth effect will be. If your subject is standing against a wall there will almost no effect at all.
Lighting is important, too. We experimented in morning sunlight for Fashion Photography with iPhone portrait & default mode and the results were rather lovely.
Shot with Iphone 7+
Edited with Snapseed and Picsart
Garments by : @Punikatms