There is no process flow difference between PM & CS for some of the processes. Plant Maintenance is for Internal / own asset maintenance, and CS is for doing any kind of service to the customer & getting revenue out of it.
I am not sure which process flow (PM or CS) you are looking for.
CS Order type may bear revenues whereas PM Order types can't generate revenues. You can make this setting while defining order type in OIOA (Revenue Allowed tick mark)
If you want to bill the customer based on sales order, not based on service order, then service order will not bear any revenue as such, its corresponding sales order will generate revenue. Service order cost will be settled to sales order.
SAP Library - Customer Service (CS)
SAP Library - Plant Maintenance (PM)
You can refer above link for each & every process flow.