Engine ID is unique identifier. It is not MAC address, but can be derived from MAC. Engine ID should be the same until reboot of the device/process.
each SNMPv3 component should have unique EngineID. You can configure it on switches what EngineID will be used.
if you use SNMPv3 traps then you need to configure the EngineID of the switch in the XMC.
If you use SNMPv3 inform then you need to know the EngineID of the XMC before you send the inform. many operating systems (including XOS) does check the EngineID of the management before sending the inform.
I guess the counter is showing how many times the EngineID was resolved before the Inform was sent.
Regards
Zdeněk Pala