Question regarding <ci>

Hello,
can anyone explain in a few words what could be a meaning of the following 
MathML piece in case of computing application:

<math>
        <apply>
                <plus/>
                <cn>2</cn>
                <ci>x</ci>
        </apply>
</math>

as far as I understand this should be rendered like 2+x
but what if rendering is not required, and the only task is the result 
computing ?

What the X actually is in this case?
Is it identifier of something declared somewhere else by <declare> 
construction or anything also?
Can <ci> identifier obtain it's value in a way other than <declare>ing ?

Thank you 
Max