DBai

Hi @DBai I understand all of that, I was really just thinking out loud. A Layer is probably just a container to keep things tidy and in itself is nothing special but a distinction has to be made between the two names and a capital L is the indicator for what I consider the container Layer.
As with other layers it might be prudent to give the Layer layer a sub name such as 
Layer (Container)  Layer (Sheet)  Layer (Board) Layer (Page) or something like that, anything but Layer (Layer).  Layer (Layer) is not really a good syntax and sounds like your consoling someone. 
My tongues going numb saying layer all the time. 

OK, it seems that Mike has discovered how to make PDF layers within AD. I did manage to do the same, the procedure is to create an empty layer and paste inside the goods. It appears as a layer in Acrobat.
 
PDF layers are a latecomer in PDF technology, normally objects appear in PDF file jus as the writing app happens to write them, usually separate but not as distinctly defined LAYERS. Usually it just does not matter, I guess layers feature was some CAD crowd idea.

I think if AD ever gets actual pages (as in more than one) it should start a new page-based document with Layer 1 and objects could only be on/in a user-specified layer.
 
That's really how APub should be and so if/when AD also gets multi-page capabilities, it ought to apply to it, too.

No, in the terminology used by Affinity, it is on its own layer (the uncapitalized generic name for any type of layer object), not its own Layer (a specific type of object 'container' layer denoted by the capital L). And considering that any layer type can be assigned a user-defined name which can be quite long, neither the "(Layer)" nor any other suffix may be visible in the Layers panel, so you can't assume it will always be clear what any type of layer might be.
 
More prominent color coding of "(Layer)" layers might help, but you still have to assign them manually to each layer, plus if the Layers panel is relatively narrow similar gradients would be hard to distinguish from one another. Besides that, the layer color is also what determines the color of the bounding box & other selected item indicators on the canvas, so gradients would not be very workable for that.

So in your screenshot, which Layer (the capital L kind) is the diamond or cog in? 

If you think everything is a layer it can be confusing but the layers are named clear enough.
 

You need to actually create Layers in the Layers Panel and place/create the objects into those layers.
 
Yeah, it's confusing because Serif saw fit to call any/every object a layer, but they are not. They are objects.

Your file has artboards, no layers.
 
The attached AD file and resultant PDF has layers.
 
Mike
4-layers.afdesign
4-layers.pdf