What is an "XForms Processor"?

Having earlier posed the question of what an XForms form may be, I now find 
myself in the position of having to ask what an XForms processor is.

In 12.1.1 by implication a conforming XForms processor may be located on the 
server.

Why would a server-side "XForms processor" need, for example, to support user 
interaction events?

If data from a client-side XForms processor is submitted as generic XML, 
whose structure is determined by the client-side instance data, what does a 
server-side XForms processor do in that situation?

Or does 12.1.1 simply need to be edited?

Andrew Watt