For a field that is null-capable in the RPG program, the following will
apply on input, for DISK, SEQ, WORKSTN and SPECIAL files:
When a null-capable field is read from an externally described file, the
null indicator for the field is set on if the field is null in the record.
Otherwise, the null indicator is set off.
If field indicators are specified and the null-capable field is null,
all the field indicators will be set off.
If a field is defined as null-capable in one file, and not null-capable
in another, then the field will be considered null-capable in the RPG program.
However, when you read the second file, the null indicator associated with
the field will always be set off.
An input operation from a program-described file using a data structure
in the result field does not affect the null indicator associated with the
data structure or any of its subfields.
Reading null-capable fields using input specifications for program-described
files always sets off the associated null indicators.
If null-capable fields are not selected to be read due to a field-record-relation
indicator, the associated null indicator will not be changed.
When a record format or file with null-capable fields is used
on an input operation (READ, READP, READE, READPE, CHAIN) and a data structure
is coded in the result field, the values of %NULLIND for null-capable data
structure subfields will be changed by the operation. The values of %NULLIND
will not be set for the input fields for the file, unless the input fields
happen to be the subfields used in the input operation.
Null-capable fields cannot be used as match fields or control-level fields.