Interpreting an XPath expression to understand a particular schema

Question

This is my first foray into XML. I'm trying to understand Java-based XML pipeline processing structure, but I'm a bit confused by the usage of XPath.. basic abbreviated structure is as follows. In particular, I am trying to wrap my head around the pipeline schema for the XNAT database.

<Pipeline>
    <documentation>
        <author>
        </author>
        <input-parameters>
            <parameter>
                <name>example_parameter</name>
                <values><schemalink>xnat:imageSessionData/ID</schemalink></values>
            </parameter>
        </input-parameters>
    </documentation>     
    <outputFileNamePrefix>^concat(/Pipeline/parameters/parameter[name='workdir']/values/unique/text(),'/DicomToNifti')^</outputFileNamePrefix>
    <loop id="series" xpath="^/Pipeline/parameters/parameter[name='scanids']/values/list^"/>
    <parameters>
        <parameter>
            <name>workdir</name>
            <values>
                <unique>^concat(/Pipeline/parameters/parameter[name='builddir']/values/unique/text(),'/',/Pipeline/parameters/parameter[name='sessionId']/values/unique/text())^</unique>
            </values>
        </parameter>
    </parameters>
</Pipeline>

If I am understanding correctly, the <outputFileNamePrefix> element is using XPath to select the value of the workdir parameter... which itself seems to be a slightly different XPath expression to a similar parameter with the name builddir, but that parameter does not seem to exist anywhere within the pipeline schema... the overall project is probably a little too involved to include in a stack exchange question, but perhaps someone could point me in the right direction...

Again, the code above has been abbreviated. The full .xml can be found here.