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Constraints guide editors to add data correctly by displaying a small icon next to the field that has a constraint. Here is a list of constraint descriptions. When they show up on an item they show up as an icon, like this image.
Interested in learning more about constraints? You can browse items by constraint violations on this page. An example of a constraint violation is distinguishing between two properties. The properties P131, located in the administrative territorial entity, and P276, location, both represent location in different ways. Constraint reports can help you discover and correct properties that are not used correctly.
If you want to know if a property requires a constraint, open this link in a new tab.