This morning ,I got a three-phase plug with 6 wire terminals( see pictures), L1 , L2, L3, N/W, Earth, and a "pilot" terminal, I don't know what 's the "pilot" terminal use for , for testing or other purposes ?

Typically to operate a contactor, controlling the supply.

Hi ,geoff, Thank you for your answer

This picture was taken in my workshop , and these three-phase plug mainly used for connecting equipment like water pump , electric welding machine to the AC 380V distribution box on the wall，no control circuit inside the distribution box , and controlling the power supply only by RCD

Well, just not used in your case, and probably has no connection, but that's what it was intended for if needed.

Perhaps an indicator light, or a monitoring device?

If it's a CEE Standard plug, the Pilot contact is for operating a Contactor, so the plug contacts don't wear off when connecting or disconnecting under load.
See here, chapter 4.1 https://en.wikipedia.org/wiki/IEC_60309

Hi, andey. This is a "MENNEKES" plug with made in Germany, I have seen the fully detailed explanation about the "pilot" contact on wikipedia website, thank you for your help!