The properties are key-value pairs used to store extra information on the menu items. This endpoint interacts with the default properties for this menu: when creating a menu item from the UI (Experience Manager), it will automatically contain the default properties. When creating menu items through the /items endpoint of this API, any property can be defined, and only explicit properties will be created (it ignores the default properties). Existing items won't be affected by changes on the /properties endpoint, if they have to be updated, the /items endpoint can be used to include/remove any property for each item.