SOAP-based services are XML based and come with a standard security mechanism (WS-Security protocol). This allows for a security element to be presented as part of a SOAP web services call. There are multiple profiles that define these standards (i.e. the username profile which uses a username and password security token or the x509 profile that uses a certificate as a security token) as an authentication token.
The WS-Trust standard introduces the concept of a Security Token Service (STS) that the web services client and the web services provider can lverage to broker the authentication. In the WS-Trust model, a security token (i.e. a SAML assertion) is issued by the STS for the web service client. This token is passed to the web services provider during the service call. The provider will validate this token against the STS and if valid, allow access to the web services call.