CrossArk Servers are a cluster of servers containing multiple maps. Say scorched earth and center. Character Transfer is only possible within this cluster between the different maps so you don't have to restart each time.
Though, you cannot transfer from one CrossArk Cluster to another. If you play on crossArk2 center you cannot transfer to crossArk1 center but only to other maps within crossArk2.
On main official servers however, you can transfer to any other main official server that has not isolated themselves from the main servers.