Why exchange rate type "EURX" is used ?

Dear all,

I post with FB65 a document with currency EUR.

The local currency is THB and group currency is CHF.

The document type doesn't have specific exchange rate type ==> by default the type "M" is taken.

But, when I post a document with currency EUR, the system doesn't take the type "M", but the type "EURX".

In this case, I don't want to use "EURX", but I want to use "M".

How can I do ?

Thanks for your help

Best regards