When transferring costs between sources of funds, it is recommended that the transfer should use the original transaction code wherever possible. However should this not be practical then the generic transfer code of EZXX can be used, but must be used on both the debit and credit side of the journal.
It is much more unusual to transfer income, indeed this is not to be done for certain sources of funds. There is a general transfer code, LXBB, for exceptional circumstances.
Further details are provided in FPM Chapter 3: Chart of Accounts and in Best Practice Guide No. 3.