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