It is user defined. Typically, you will have power rails in Mx, and Mx+1. Then clock starts at Mx-1. This way you can isolate some of the coupling and restrict some of it to a single layer.
CTS is suitable for which metal layers?n for timing clear what layer should use???how particular metal layers are defined for certain things like cts,routing,timing..???
CTS is suitable for which metal layers?n for timing clear what layer should use???how particular metal layers are defined for certain things like cts,routing,timing..???
Depends on the metal stack, but it is usually pretty easy to identify the wider metals that are meant for power. everything else is signal, and therefore can be used for clock tree.
All tools have configurations that you can play with to specify preferred layers for routing, for clock routing. Not sure what you mean by timing, all layers can be timed.