On normal Officials, you can transfer from any map of one game type to another map of the same eg PvE Official Island to PvE Official Ragnarok to PvE Official Center.
On clustered maps, you can only transfer between servers on that specific cluster. So for example there might be cluster 1 which has one of each map, so from IslandCluster1 you can only transfer to CenterCluster1 or RagnarokCluster1. They're grouped servers with locked down transfers
You can also do this on unofficial servers - dedis can have all transfers enabled allowing anyone from any other dedi with them enabled to transfer to their server, they can turn transfers off entirely, or they can host a cluster where people can use more than one map but within one server 'family' as it were
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