If the Search Manager can assign a citation of type "unknown" to another citation type without losing an data, it should do so. Maybe it should find the best match by looking at the fields related to "source" (pub place, which is mostly relevant to books, for example, and volume/issue, which are mostly relevant to journals), but also determine whether any fields are present that would be lost by assigning to a type that would not be lost by leaving as "unknown".