Please read the docs:
Can be undefined or set to "false", "true", "a_only", "b_only"
false - indicates to not process the record.
true - or undefined indicates the default behavior which is to process all CDR records.
a_only - indicates to only process CDR records on the inbound leg of a call.
b_only - indicates to only process CDR records on the outbound leg of a call.
Usage:
<action application="set" data="process_cdr=a_only"/>
If you do a_only or false, that is supposed to override also logging the B leg, even if you set it with mod_xml_cdr.