The choice between a one-way and round-trip dummy ticket looks like a pricing question. Usually, it's an itinerary question. Are you trying to show when you'll leave a country? Or do you need a proposed journey that explains both your arrival and departure?
Choose the wrong format and you can end up with a perfectly neat PDF that answers the wrong question. Choose the right one and your travel plan becomes much easier to understand.
Here is a practical way to decide without buying extra flight segments simply because they look more official.
What a one-way dummy ticket is good at
A one-way temporary flight reservation shows a single planned flight, often from the country you are visiting to another destination. It is especially relevant when you already have an inbound flight but need separate proof of onward travel.