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For this, choose the axis or ordinate, click the 3 dots next to the element and select Edit.
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Optional key
"This key is optional" is an attribute available only for open or semi-open axis' keys. This attribute enables data architects to mark some of the keys are not mandatory to fill in for every row/column in the template.
This property is reflected in XBRL taxonomy table linkbase as includeUnreportedValue attribute or asOptionalKey (BOOLEAN) column in the mAxis table when the model is exported as a database.