Description

   You can "undo" patches only in reverse chronological order of the
   creation dates of their recovery data sets. Thus, if you install
   patches A, B, and C with separate PRODUCT INSTALL commands,
   and you want to "undo" patch B, the PRODUCT UNDO PATCH command
   requires that patch C be removed along with patch B. To remove
   complete products, including any patches or mandatory updates
   that have been applied to them, use the PRODUCT REMOVE command
   instead.

   A recovery data set contains a copy of the directories, files,
   and libraries that were modified or deleted by the execution
   of a PRODUCT INSTALL command. In addition, a snapshot of the
   product database is included, along with other information needed
   to restore the software environment. Each recovery data set is
   stored in a directory tree on the system device in [PCSI$UNDO_
   nnn...], where nnn is a number from 001 to 999. The most recent
   data set is number 001; all data sets are renumbered when a new
   data set is added or deleted from the stack.

   The PRODUCT INSTALL command creates a recovery data set when
   patches (also known as remedial kits or ECOs) are installed
   unless you disable this feature with the /NOSAVE_RECOVERY_
   DATA qualifier. A recovery data set contains information about
   all patches that are installed concurrently. The PRODUCT UNDO
   command removes all the patches that are in the selected recovery
   data sets. Therefore, if multiple patches are installed in one
   operation, they can be removed only as a group.

   You can use the PRODUCT SHOW RECOVERY_DATA command to list all
   recovery data sets and find out which patches can be removed. To
   delete unwanted recovery data sets, use the DELETE RECOVERY_DATA
   command.