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.