* The subject's pose and expression: A wider horizontal format might better suit a subject looking off to the side or engaged in an action.
* The background: If the background is important to the story or context of the portrait, a horizontal format might allow you to include more of it.
* The intended use of the portrait: A social media profile picture usually favors a portrait orientation to be seen correctly on a phone screen, a large landscape print might suit a horizontal aspect ratio.
* The overall aesthetic you're aiming for: Horizontal formats can sometimes feel more relaxed and cinematic.
In conclusion, portrait formatting is a good starting point for portraits, but it's important to consider the specific circumstances and artistic goals before deciding on the final format. Experimenting with both portrait and landscape orientations can lead to more creative and impactful portraits.