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A Root Map (or master map) specifies a DITA map that defines a hierarchical structure of submaps that are contained within the root map. Essentially, the root map defines a scope and provides the mechanism to allow your defined keys to be propagated throughout the entire map structure (this mechanism is also known as a key space).

In Oxygen XML Author Eclipse plugin, the DITA Maps Manager includes an option on its toolbar where you can easily specify the root map, but there are also several other ways to select or change the root map.