Creatures that are dueling with each other are in a liminal space between the Material and Ethereal Realms. This place is most often called the "Duel Space". While dueling, the creatures cannot get more than 30 feet apart nor interact with anyone but each other and creatures that are not dueling cannot interact with ones that are. Dueling always occurs between two or more creatures. There can be multiple groups of dueling creatures on the battlefield at the same time, but those groups cannot interact with each other. Creatures can only interact with those that they specifically are dueling with.
A dueling creature that becomes incapacitated has "lost the duel" and returns to the Material Realm. This ends the dueling condition. If there is only one creature remaining, then they choose whether to return to the realm they were in prior to dueling or to the Material Realm.
Any Dueling creature may use Leave Duel to attempt to stop Dueling. In this case, they count as "losing" and return to the Material Realm.
Some effects, such as Ninja Duel, may alter the rules that Dueling operates under. These could apply either when Dueling begins or while it is already underway.