Yes. WRX transaction event key is triggered during MIRO transaction.
WRX transaction key is GR/IR clearing account.
When you will do a GR (MIGO), then system will credit the G/L account which has assigned with valuation class in WRX transaction event key.
At the time MIRO, system will debit the amount from the same G/L account which has assigned with valuation class in WRX transaction event key.
Please search in SCN to read more about WRX transaction event key.