[Qgis-developer] What is "Discourage other labels from covering features in this layer" option in layer properties?

[DelazJ]

In QGIS master (524716f), Layer's Properties > Labels, where we use to
select in a combobox the field to label, there is now a combobox with
options like :
- No labels;
- Show labels for this layer;
- Discourage other labels from covering features in this layer.
I'd like to understand why this third option is there and what it does
(since choosing it just deactivates labels properties - just like "No
labels" option).
If it's supposed to set the layer features as obstacles for other layer's
labels, shouldn't it be a checkbox in Labels > rendering > Obstacles?
Moreover, someone may want to show labels of a layer but discourage other
layers to have their labels covering the features of the first layer. Is
that possible with this configuration without duplicating the layer?

Thanks
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