An External Account Document is a document uploaded by the customer to help prove they own an external account . Examples include an image (or pdf) of a bank statement or a driver's license. This object is only used in rare circumstances, e.g., a customer has reached the maximum number of allowed external accounts and they need to swap an existing external account for another one.

Take me to the External Account Document Endpoint Reference

externalAccountDocument Object

Property	Data Type (length)	Description
`customerId`	integer	The Helix-assigned unique ID for the customer who uploaded the document.
`documentContent`	string (10MB)	A base64-encoded representation of the actual content of the document. NOTE: Applicable only when POSTing to the /externalAccountDocument/upload route. This property will not be returned by any route. Base64-encoding should be done using the RFC 4648-compliant encoding (i.e. no embedded line breaks and use `=` as the padding character). See RFC 4648 for more information.
`documentName`	string (50)	A caller-specified, user-friendly name for this document. Maximum length is 50 characters. Must have one of the following extensions: `.jpg` `.jpeg` `.gif` `.png` `.pdf`
`documentType`	enum	The type of document. Possible values include: `DriversLicense` `BankStatement`
`externalAccountId`	integer	The Helix-assigned unique ID for the external account to which this document is associated.
`reasonType`	enum	The reason for uploading the document. Possible values include: `NameChange` `AddressChange` `Unspecified`