Auto matches the shape of the drawing you uploaded. The Render page offers six settings in total and accepts nothing else: Auto, 16:9, 4:3, 1:1, 9:16 and 3:2. There is no field for a custom ratio, and a value outside that list is refused with "Invalid aspect ratio".
What Auto actually does
Auto measures the width and height of your uploaded image and picks the nearest standard shape. The list it picks from is wider than the dropdown you can see: as well as the five ratios in the menu, it can choose 3:4 or 2:3, so a tall drawing comes back tall even though 9:16 is the only portrait option offered to you.
Nearest is not exact. A drawing at an unusual ratio is rendered at the closest of those seven rather than at its own proportions, so expect a small change of framing rather than a match to the pixel.
When to set one instead
Set a ratio when the output has to fit something: a 16:9 slide, a 1:1 post, a 9:16 story. Setting one overrides the shape of your drawing, and the render is composed to the ratio you asked for.
Auto reads your drawing, never the style reference
The measurement is taken from the main image only. That is deliberate: a style reference in a different shape would otherwise drag the output to its proportions instead of yours.
Virtual Staging has no ratio control
The Stage page always renders at Auto and offers no setting at all. If you need a fixed shape out of a room photo, crop the photo to that shape before you upload it.