The Synthetic Domain (or Domain) is a parallel universe which sits directly on top of the mortal universe (referred to as the mortal realm). It is where the Lightbearers originate.
The main purpose for the Domain is for the Lightbearers to reside while not partaking in the Tournament. Because they are ethereal spirits, and after their possession of a mortal has ended, their spirit itself cannot die. Instead, as soon as they have lost control of their mortal body, their gaseous manifestation is pulled back — similar to a magnetic force — into the Synthetic Domain.
Inside the Domain itself, it is impossible for physical objects to exist. When Ramor created the Domain, her intention was solely to allow for the retreat of her Children of the Ethereal. However, Ascended Lightbearers have the ability to temporarily return to the Domain, using a specific item found in the mortal realm. They may also bring non-Ascended with them, if they are attached to the same item.
While Lightbearers are present in the Domain, they can only communicate via the Ethereal language and via the Synth. The presence of others can only be made aware if they choose to announce themselves. If they are called into the Domain via an Ascended and still have possession of a mortal body, this body appears to freeze in time but it does not die.
Additionally, Ramor is the only entity that can exist in both the Domain and the mortal realm. This is so she can watch her Children and communicate with the Ascended to adjudicate the Tournament.
It is important to identify what is referred to as the mortal realm. Because Ramor is the star central to her own planetary system, this does not mean that these worlds exist in a completely different reality.
Space is the known infinite and with distance being the major obstacle, the mortal realm is the same plane of existence that Earth resides. Ramor may be in a completely different galaxy — of which the universe contains many — but this does not have to imply complete separation.
Also, being parallel, an exact location in the mortal universe equates directly to the same position in the Synthetic Domain.