The initiator of the communication chooses which method of indirect communication is appropriate depending on capabilities, message size, or other external factors.
Browser extensions or other software that support Open ID Authentication may not detect a Relying Party's support if this attribute is not set appropriately. All messages that are sent as HTTP requests (GET or POST) MUST contain the following fields: Arbitrary precision integers MUST be encoded as big-endian signed two's complement binary strings. When a message is sent as a POST, Open ID parameters MUST only be sent in, and extracted from, the POST body. The form field's "name" attribute SHOULD have the value "openid_identifier", so that User-Agents can automatically determine that this is an Open ID form. To initiate Open ID Authentication, the Relying Party SHOULD present the end user with a form that has a field for entering a User-Supplied Identifier.