The so-called "Imperial Races" are those cultures conquered and "tamed" by the Tarn Empire. For nearly a thousand years, their cultures were greatly influenced (if not almost completely altered) by the conquest and occupation by the Tarn.
These independent cultures were never conquered by the Tarn Empire, and thus cannot be considered "Imperial Races". However, for a variety of reasons (being powerful enough to be considered a rival of the Empire, being cultured enough to be considered "civilized" despite not being a part of the Empire, simply never encountering the Empire at all), they were not labelled "barbarians" by the people of the Empire, or simply just not being labelled in any way.
The so-called "Barbarian Races" are those cultures existing (for the most part) on the borders of what was once Imperial territory. They were called "barbarian" by the Imperials because of the common view that anyone not a part of the Empire was... well... a barbarian.
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