Re: impedance.
Impedance is the one of the most important characteristics of PCB layout design. If your impedance is not controlled, the output characteristics of the signal will not be the same as you expect. For high speed design, impedance is the major concern.
When you doing a Split planning, you need to avoid the traces crosed between two planes. In this, the impedance of the trace is mismatched in the anti-etch area. So you can't get the desired output.
Manikandan