Did you know how RC/Merger use the REGION size ?
If RC/Merger knows the storage requirement for a particular job it can include the REGION parameter. If RC/Merger are not sure of the storage, it can simply ignore the REGION parameter or set the REGION parameter to 0. So that, maximum storage can be obtained for a Job based on the availability.
If RC/Merger job requires more virtual storage based on the number of lines to process the setting of the region parameter at the job level is suggested (REGION=0 or ignore the REGION parameter).