The tag allows control for the action attribute of the form.
Every JSF request goes to the same url which renders the response. The s:form component allows to use a different scheme, servername or port when a form is submitted. This is normally needed to change from http to https to submit security related data.
Even more, by setting an entirely different 'action' and 'method' attribute the standard behaviour of the JSF-Form can be overwritten completely. This might be useful for special cases of forms, e.g. for form based authentication.
servername - the server name or an ip address (optional)
port - a int value which defines the port (optional) method - a method with which the request should be sent (optional) action - if this attribute is set, the action of the form can be specified directly. Scheme, servername and port are not used in this case (optional).