To call one of the webservices methods, you need to be authenticated. With the other SOAP interfaces, this is done using a SOAP header, which contains either a session token or a username and password.
Some clients, however, do not support sending SOAP headers. The headerless SOAP interface makes it possible to use SOAP, without using SOAP headers. Authentication is done by sending extra parameters.
The SOAP “no headers” endpoint takes a session token (reactid) as its first parameter.
The SOAP “simple” endpoint takes the username and password as its first and second parameter.
Other than its parameters, this endpoint uses the same protocol as the SOAP RPC/Encoded interface.