An input-only field can be used for read operations only. Your program can read data from the field, but cannot update the field in the file.
Typical cases of input-only fields are key fields (to reduce maintenance of access paths by preventing changes to key field values), sensitive fields that a user can see but not update (for example, salary), and fields for which either the translation table (TRNTBL) keyword or the substring (SST) keyword is specified.
If your program updates a record in which you have specified input-only fields, the input-only fields are not changed in the file. If your program adds a record that has input-only fields, the input-only fields take default values (DFT keyword).