External Invoice FX, what is the proper Billing type for external credit note?

Hi Gurus,

We are using external invoice type FX

The invoice is created with BAPI_BILLINGDOC_CREATEMULTIPLE.

Now we are looking for a external credit note invoice type.

I would have expected GX, but there is no such one.

Does anyone know the proper credit note type, which we can use for external?

Als is there a special invoice cancel invoice type?

THANKS

Regards

Dieter