Field in IFS to group combinations of Account and Code_B

Do you mean to hardcode a code part value based on the value ? If so, that is controlled in Code String Completion window. The below example would mean that whenever Account 1010 is used, the cost centre will be hardcoded to 101. 

This can also be controlled through the setting on posting control from the “Override” field (if set to Yes - the Code String Completion will be taken into account, else it will be whatever is set up in Posting Control). 

If you would like to combine 2 different code parts to equal a third code part or a different field altogether and then use this for reporting purposes, like an attribute, then you would have to look at a new configured table to do this mapping. 

e.g. Account 1234 and Region - West combination should always be assigned a reporting attribute/code part X, but account 1234 and region - South should be attribute/code part Y etc.

Reporting attributes and reporting structures, and code string completion is currently only assigned to a single code part out of the box. But it would not be a complicated configuration to extend the mapping to 2 code parts.