1) What is the difference between Automation Engineer & PLC Engineer. What's the scope between these two?
2) What is the difference between HMI & SCADA?
3) What is the difference between PLC & DCS?
PLCs are used in automation. Automation is a lot more than PLCs alone, you can also have relay logic, electronic control cards, drives etc. Apply this to the engineer and you'll see that an automation engineer has to learn a lot more than only PLC. A PLC engineer on the other hand should have a far greater knowledge on PLC than an automation engineer.
HMI is Human Machine Interface. SCADA is Supervisory Control and Data Acquisition. In other words: HMI is limited to just interfacing, SCADA can be a lot more.