The user defined filter facility provides a mechanism
  whereby filters written as DEC Text Processing Utility
  (DECTPU) programs are automatically executed when tests
  are run. These filters are referred to as user filters.
  The implementation enables users to solve easily many
  filtering problems, often with a single-line program,
  whilst allowing full access to the facilities of DECTPU
  to solve more complex cases.

  To implement a new filter, a file containing the
  required DECTPU commands is created. There are a
  number of predefined patterns and a global replace
  procedure provided which can be used to build the
  commands. For example, the following command will
  replace device names that precede a directory with
  the string "DEVICE":

     global_replace( identifier + ':[' , 'DEVICE:[' )

  Procedure global_replace is similar to the TPU pattern
  style feature of Language-Sensitive Editor and filters
  can be developed using that facility.

  To associate a user filter with a test, a logical
  variable starting with the characters "DTM$UF_"
  is created. The value of the variable is the file
  specification of the file containing the DECTPU
  commands.

  The variable is then associated with the test in the
  usual way.

  When the test is run, as part of a collection, the
  filter will be applied.