Allows to tie features across symbolizers, styles and layers. Once two features are connected, their mutual position and other properties can be adjusted.
Every symbolizer can have key property, whose value will be unique feature id in most cases. For example this MarkersSymbolizer:
<MarkersSymbolizer key="[id]" />
Another symbolizer can connect to the marker by anchor-key property. In this case of simple point text placement, dx and dy properties of the text symbolizer will be set according to the size of the marker so one doesn't have to adjust these properties if size of the marker changes:
<TextSymbolizer
placement="point"
placement-type="simple"
anchor-key="[id]"
>
</TextSymbolizer>
Whereas all symbolizers can set their key property, only point symbolizer and markers symbolizer can offer its size and only text symbolizer can receive it by anchor-key currently.
However, anchors are meant to be universal tool for connections between features and more possibilities will come.
