You can use logical names to save keystrokes if you work  in  several
  dictionary  directories  with long path names.  With the DCL commands
  DEFINE and ASSIGN, you can define logical names for  CDD  path  names
  you  use  often.   Use either of the following formats in response to
  the DCL dollar-sign prompt:

  DEFINE logical-name "_CDD$TOP . . . given-name"

  ASSIGN "_CDD$TOP . . . given-name" logical-name

  Once you have defined logical names, you can use  them  in  place  of
  path  names  in utility command lines.  The CDD attempts to translate
  the first given name of any path specification  as  a  logical  name.
  For  example,  if  you  specified  the path name SALES.JONES, the CDD
  would make one attempt to translate SALES.  If SALES were not defined
  as  a  logical  name, the translation would fail, and the CDD utility
  would process the directory CDD$TOP.SALES.JONES.

  If, however, SALES were defined as a logical  name,  the  translation
  would  succeed,  and  the CDD utility would attempt to process a path
  name beginning with SALES' translation string.

  To prevent logical name translation, prefix  the  path  specification
  with  an  underscore (_).  The CDD would make no attempt to translate
  SALES if you specified the path name, _SALES.JONES.

  Instead, the CDD would process CDD$TOP.SALES.JONES