And then there's the 7 layers of the osi model...though only four OSI questions use it...
layer looks to cluster with a lot of other vague terms like position scene creation :-/
I don't care for these tags. They're likely to only be used by the initial asker. StackOverflow curators wouldn't add them to point the OP to an easily-browsable set of questions which could broaden their horizons on the problem space they are currently addressing.
To my mind is the exact opposite situation of a taxonomizing tag, such as one I liked which was challenged: playing-cards
How should questions dealing with playing cards be tagged, or should they?
There you can address situations where people ask a question going:
"I'm writing a poker game in Java..."
...but then go on to talk about some issue on representing hands or something. A day later they're back with:
"In my poker game in Java, I want people to be able to drag a card from the deck..."
Had they been pointed at all the [java]+[playing-cards] questions by a re-tag on the first question, they might have some benefits.
But as for layer, position, scene, creation...a text search can probably find them as well as anything. They're incidental words that probably happened to be in the question or title, and if they were not then no one would have thought to add them as a tag.
I'd propose starting with
Killing those instances used with osi, it's implicit
Remove it from any graphics or CSS-related question that had been explicitly tagged with z-index or z-order (Sidenote, those look like they should be synonyms)
If a graphics or CSS question used it and was actually talking about the general concept of Z-ordering (and not some specific technology with a more abstract idea of RenderingLayer, AnimationLayer, GameLayer), tag with z-index
Remove it from any closed questions, such as those about how to make a new layer in Photoshop
...and then see what's left in terms of meaningful taxonomy classes.