Else If". But if you have many commands, with "If..Else If" you will lose overview easily. In this case switch...case would be more convenient. Sadly Arduino IDE does't support switch with String ...

How to use switch...case on strings in Arduino IDE If you have just a few ascii commands to handle, received by Serial, it's okay to use "If...Else If". But if you have many commands, with "If..Else ...