The proxy may be configured to accept, reject or convert records. When accepted, the target passes search/present requests to the backend target under the assumption that the target can honor the request (In fact it may not do that). When a record is rejected because the record syntax is "unsupported" the proxy returns a diagnostic to the client. Finally, the proxy may convert records.
The proxy can convert from MARC to MARCXML and thereby offer an XML version of any MARC record as long as it is ISO2709 encoded. If the proxy is compiled with libXSLT support it can also perform XSLT on XML.