The edit string character ? (a question mark) is the missing
    separator character. If the field has a missing value attribute
    or relationship, this character separates two edit strings. If
    the field value is not the missing value, the first edit string
    controls the output of the field. If the contents of the field
    contain the missing value, the second edit string controls the
    output of the field.

    CDO Edit       999?"Unknown"
    String:
    DTR Edit       999?'Unknown'
    String:
    Field Value:   missing value
    Edited         Unknown
    Value: