Earth Prime is the common consentual form of Earth used by the greatest number of roleplayers in the Nations Universe. Its borders can shift somewhat from story to story to accomadate given players and maps, but it is commonly agreed that, unless otherwise specified most plots and nations exist in the same world.
Earth Prime differentiates from possible alternate dimensions for iterations of this planet. Some groups, such as the 40K Power Imperium of Man accepts that their "Terra" is an alternate form of Earth elsewhere in space or time.
There are frequently other additional Non-Canon Roleplays which take place on "Earth," but do not affect Nation's history outside of the plot, making it unimportant to establish them on Earth Prime.