Complex sentence: The statuses of core and periphery are not exclusive and fixed geographically; instead they are relative to each other: there is a zone called'semi-periphery' which acts as a periphery to the core and a core to the periphery.
Split into simpler sentences: The statuses of core and periphery are not exclusive and fixed geographically, but are relative to each other. A zone defined as'semi-periphery' acts as a periphery to the core and as a core to the periphery.