Custom external data allows a client application to add app-specific metadata to Tasks in the API. The custom data includes a string id that can be used to retrieve objects and a data blob that can store character strings.
The blob may store unicode-safe serialized data such as JSON or YAML. The external id is capped at 1,024 characters, while data blobs are capped at 32,768 characters. Each object supporting external data can have one id and one data blob stored with it. You can also use either or both of those fields.
The external id field is a good choice to create a reference between a resource in Asana and another database, such as
cross-referencing an Asana task with a customer record in a CRM, or a bug in a dedicated bug tracker. Since it is just
a unicode string, this field can store numeric IDs as well as URIs, however, when using URIs extra care must be taken
when forming queries that the parameter is escaped correctly. By assigning an external id you can use the notation
external:custom_id to reference your object anywhere that you may use the original object id.
Note: that you will need to authenticate with Oauth, as the id and data are app-specific, and these fields are not visible in the UI. This also means that external data set by one Oauth app will be invisible to all other Oauth apps. However, the data is visible to all users of the same app that can view the resource to which the data is attached, so this should not be used for private user data.