Sometimes your data includes the definition of custom territories. For example, let's say your data had a field named Region that already grouped zip codes into various regions. That is, every zip code was contained in only one region. You might not want to take the time to select marks and group them manually.
Instead, you can tell Tableau the relationship already exists in the data. In this example, you'd use the drop-down menu of the Region field in the data pane and select Geographic Role | Create From... | Zip Code. Region is now a geographic field that defines custom territories:
Use ad hoc custom territories to perform quick analysis, but consider field-defined custom territories for long-term solutions because you can then redefine the territories in the data without manually editing any groups in the Tableau data source.