Chart of Accounts Mapping in Case of Primary to Secondary Ledger

While doing Chart of Account Mapping between Primary Ledger (Source) & Secondary Ledger (Target), if Source CoA has more segments than Target CoA, how the mapping will be done? Though the mapping definition is primarily driven by Target CoA, means mapping should be done for all the Target CoA Segments to the minimum, how system will consider the left over segments/associated code combinations in Source CoA in such cases? Any Pointers?

For Example: Target CoA has 4 Segments where as Source CoA has 5 Segments !