For PortalItem, this method updates the item's properties on the portal asynchronously. This update item operation allows users to update item information without changing the item content. Users can use this operation to update item information such as the title, description, tags, thumbnail, etc. This call is available to the item owner and the administrator of the organization.
For LocalItem, this method updates the item's properties in a file on disk where applicable. Updating properties can be only done for local items that are unpacked. For example a mobile map package file (.mmpk) can be unpacked using UnpackAsync(String, String). Once unpacked, local items within the package can be updated see Item and for any maps use Item. As well as packages, the local item available from an ItemResourceCache can be updated. This is accessed with Item. If the package is not unpacked an exception is thrown by the async task.