Up to now, you have been able to use flat structures as arguments of ABAP statements wherever single fields of type C were expected. In a UP this is no longer generally permitted. In a UP, you can use a structured field in a statement expecting a single field only if this structured field consists of character-type elementary types or purely character-type substructures. The structure is treated like a single field of type C.

The main restrictions applying to a UP in contrast to an NUS result from the fact that flat structures are only considered character-type on a limited basis, and fields of type X or STRING are never considered character-type. In addition, flat structures are only considered numeric-type if they are purely character-type. Numeric-type arguments include, for example, offset or index specifications as in READ TABLE ... INDEX i. The following examples show a structure that is character-type and a structure that is not:

BEGIN OF struc1,
  a(2) TYPE C,
  b(2) TYPE C,
  x(1) TYPE X,
  i    TYPE I,
END OF struc.

BEGIN OF struc2,
  a(2) TYPE C,
  n(6) TYPE N,
  d    TYPE D,
  t    TYPE T,
END OF struc.

Another example is a control break in an internal table, triggered by the AT keyword. In a NUS, fields of type X to the right of the control key are treated as character-type, and are thus filled with an asterisk. In Unicode systems, conversely, the same type is filled with its initial value.