In each Expeditionary Corps building you can specify coordinated of the Kingdom that you want to explore.
After clicking the "Start" button in the building, Explorers from the building will begin to explore the specified Kingdom square by square.
In case an Explorer dies in the course of Automatic Exploration, the square where he died will be explored at the end and only after that the Automatic Exploration will be completed.
In case someone's City disappears from the Kingdom Map in the course of Automatic Exploration, the empty square will be explored at the end, and only after that the Automatic Exploration will be completed.
Several Explorers from different Expeditionary Corpses that automatically explore same Kingdom - will not cross and duplicate each other in explorations.