Layer icon question

[nomnex <nomnex gmail com>]

What's the function/purpose of the icon next to the eye icon, in the
layer window?

I find no mention of it in the doc.

eye=visible/hidden layer
highlighted = selected layer
icon in between = ?

I can click it for all the layers, but I don't see any change
either on/off.

Thanks.

-- 
nomnex <nomnex gmail com>
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