The actual usage of the mappings in support of technical requirements. The mappings provide the information to build a ‘crosswalk' that can be used to systematically convert COA data to FDM data and FDM data back to COA data, where possible. Reverse crosswalks can be problematic if multiple COA data elements are being used to derive a single FDM data element, or multiple values for a single COA element are mapping to a single FDM data element value. The ability to recreate that reverse mapping is not possible if multiple Legacy COA element values are mapped to a single FMD value.