It is used to read a particular segment occurence and hold it for update.
This call do not replace or insert anything but only hold that particular segment occurence for further processing on that record.
It is used to replace the specified segment occurence with the new one specified in the segment IO area. For replace to happen the application must have first read and held that segment for update(GHU,GHN,GHNP).
No qualifications allowed on a replace. IMS replaces the segment which has been held.