There is no process flow difference between PM & CS for some of the processes.
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)
You can refer above link for each & every process flow.