Variables with names beginning with the string "DTM$UF_" are used to
   control user filters. These variables must be logical variables but
   can be global or local.

   When tests are run that have associated variables whose names begin
   with the string "DTM$UF_", Digital Test Manager will apply the user
   filters contained in the files referenced by the value of those
   variables. Only a single file may be referenced by each variable.

   The specified files are executed by DEC Text Processing Utility
   (DECTPU). If more than one user filter variable is associated with a
   test, the files are executed in the lexicographic order of the
   variable names. The user filters are applied before any built-in
   filters that are also specified for the test.

   User filter files can be located either in OpenVMS directories or in
   Code Management System (CMS) libraries. Files may be
   specified using logical names including logical names that specify
   search lists. Wildcards cannot be used. For files in CMS libraries,
   the most recent generation on the main line of descent is used.

   Before the first file is executed the file to be filtered is read
   into the DECTPU buffer "filter_buffer". Next, the file specified by
   the logical name DTM$UFDEFINES is executed. The system logical name
   DTM$UFDEFINES references the file SYS$LIBRARY:DTM$UFDEFINES.TPU,
   which contains definitions of a global replace procedure and
   patterns which can be used in building filters. This logical can be
   redefined to point to a custom file.

   Any errors in accessing the user filter files or in executing the
   DECTPU commands will be reported. However, they will not cause the
   filter operation to fail, and any remaining user and built-in
   filters will be applied.

   After all the user filters have been applied, the file being
   filtered will be written out. If any built-in filters are also
   specified, they are applied to the newly created file, resulting in
   a second new version.

   In order to disable a user filter that is defined with a global
   variable for a particular test, define the value of the variable
   for the test as a string containing only spaces. For example:

     MODIFY TEST CHECK_DEVICES /VARIABLE=DTM$UF_DEVICE=" "