Subfile records contain a “change” flag which is turned on when the user makes a change
The READC operation reads “changed” records
If the SFLNXTCHG indicator is turned on and the subfile record is written or updated the change flag will be forced on so in the subsequent processing the READC will get that record.
One use for this is so that the records can be edited for errors and if no errors are found then they can be processed. As the readc loop edits for errors it updates each subfile record with the SFLNXTCHG indicator on and that allow the update loop to use the READC to get the records again.