An arc is simply a cicle where only a portion of it is visible.
However you don't need if for you field designs since you can configure a singe field this way.
<field name="Textfeld1" y="9.525mm" x="6.35mm" w="37mm" h="12mm">
<ui>
<textEdit hScrollPolicy="off">
<border>
<edge thickness="0.2mm"/>
<edge presence="hidden"/>
<edge presence="hidden"/>
<edge presence="hidden"/>
<corner thickness="0.2mm"/>
<corner/>
<corner/>
<corner/>
</border>
<margin/>
</textEdit>
</ui>
<font typeface="Arial" kerningMode="pair"/>
<margin topInset="1mm" bottomInset="1mm" leftInset="0mm" rightInset="0mm"/>
<para vAlign="middle" widows="2" orphans="2" marginLeft="2.835pt" marginRight="2.835pt"/>
<caption placement="top" reserve="3mm">
<value>
<text>Textfield</text>
</value>
<font typeface="Myriad Pro" kerningMode="pair" size="8pt"/>
<para vAlign="middle" marginLeft="2.835pt" marginRight="2.835pt"/>
</caption>
<border>
<edge thickness="0.2mm"/>
<corner presence="hidden" thickness="0.2mm"/>
<corner presence="hidden" thickness="0.2mm"/>
<corner thickness="0.2mm" join="round" radius="5mm"/>
<corner presence="hidden" thickness="0.2mm"/>
</border>
</field>
… to get this text field design.