OptionaldateA string used with date fields to specify how the date should appear in pop-up windows.
OptionaldigitA Boolean used with numerical fields. A value of true allows the number to have a digit (or thousands) separator when the value appears in pop-up windows. Depending on the locale, this separator is a decimal point or a comma. A value of false means that no separator will be used.
OptionalplacesAn integer used with numerical fields to specify the number of supported decimal places that should appear in pop-up windows. Any places beyond this value are rounded.
The format object can be used with numerical or date fields to provide more detail about how values should be displayed in web map pop-up windows.
See https://developers.arcgis.com/web-map-specification/objects/format/ https://developers.arcgis.com/web-scene-specification/objects/format/.