SHR allows others to read, write and update an open file. An individual record within this file may be locked during use when either OUTPUT or OUTIN is also specified and when the I/O statement utilizes the RESERVE parameter (RESERVE is the default, so explicitly coding it is not necessary). SHRU operates the same as SHR, but it is used only to maintain System/23 compatibility.
SHR allow others to freely access and update the file being opened. Use of this parameter maximizes the multi-user system's capabilities; however, it also increases system overhead (see Automatic Record Locking for ideas on reducing system overhead).
Unless otherwise specified, individual records within a file that is opened for SHR,OUTIN are automatically locked as they are read. See Record Locking for more information.