The handling of currencies in R/3 is as follows:
The currency type which you could enter in the customizing of the document type (Field T003-KURST) is only used in the conversion of the document currency to the first local currency.
For the conversion into second and third currency the specification of the transaction OB22 are taken into account.
The settings in transaction OBA7 only take effect for online postings, means FB01-like transactions that use the SAP module pool SAPMF05A. Postings from SD or MM use the so-called accounting interface. The accounting interface does not allow an alternative rate type to be used.
It always uses type M for translation to first local currency and the rate types specified in transaction OB22 for translation to further local currencies unless these values have already been supplied by the leading application.