The anchor widget has not a screen representation. It is used to create a virtual rectangle, where you can place widgets into. All widget coordinates are relative to the anchor position.

There is only one additional attribute to common widget attributs

Below you see an example for an anchor

<element name='Anchor Left Buttons' type='ANCHOR' posx='%10' posy='of:totop:Anchor BottomGap' width='%19'  height='of:width:self:*2.8' >
<xi:include href="$var(DEFINITIONPATH)/block_buttons_left.xml" parse="xml" />
</element>