Yes, that's true; in that case, it would just be a collection containing a single planar surface.

A multi-surface char source would look like this:
<pre>
        <characteristicFaultSource id="7" name="characteristic source, multi surface" tectonicRegion="Volcanic">
            <truncGutenbergRichterMFD aValue="-3.6" bValue="1.0" minMag="5.2" maxMag="6.4" />
            <rake>90.0</rake>
            <surface>
                <planarSurface strike="0.0" dip="90.0">
                    <topLeft lon="-1.0" lat="1.0" depth="21.0" />
                    <topRight lon="1.0" lat="1.0" depth="21.0" />
                    <bottomLeft lon="-1.0" lat="-1.0" depth="59.0" />
                    <bottomRight lon="1.0" lat="-1.0" depth="59.0" />
                </planarSurface>
                <planarSurface strike="20.0" dip="45.0">
                    <topLeft lon="1.0" lat="1.0" depth="20.0" />
                    <topRight lon="3.0" lat="1.0" depth="20.0" />
                    <bottomLeft lon="1.0" lat="-1.0" depth="80.0" />
                    <bottomRight lon="3.0" lat="-1.0" depth="80.0" />
                </planarSurface>
            </surface>
        </characteristicFaultSource>
</pre>
A char source with a single planar surface would look like this:
<pre>
        <characteristicFaultSource id="7" name="characteristic source, multi surface" tectonicRegion="Volcanic">
            <truncGutenbergRichterMFD aValue="-3.6" bValue="1.0" minMag="5.2" maxMag="6.4" />
            <rake>90.0</rake>
            <surface>
                <planarSurface strike="0.0" dip="90.0">
                    <topLeft lon="-1.0" lat="1.0" depth="21.0" />
                    <topRight lon="1.0" lat="1.0" depth="21.0" />
                    <bottomLeft lon="-1.0" lat="-1.0" depth="59.0" />
                    <bottomRight lon="1.0" lat="-1.0" depth="59.0" />
                </planarSurface>
            </surface>
        </characteristicFaultSource>
</pre>

Is this correct?