It depends at what point of its history the kingdom was referred as Syria. Eventually the Seleucids ended up only with Syria. But even before that their core regions were Mesopotamia and Syria. Their rule in Asia Minor and Iran was never strong and consistent. Culturally they were closer to the more Hellenized population of Syria than to more "Eastern" population of Mesopotamia.
I'm by no means an expert, so I can be wrong, so any corrections are welcome :)
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