VMS Help DECset, Commands, SET  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The SET command allows you to set the current `context' or the
  current context database for your process.

  A `context' is a named set of values that tailors the DECset tools
  to a specific software development activity (for example, fixing
  bugs or implementing enhancements).

  The SET command line has one of the following forms, depending upon
  whether a context or a context database is being set:

       DECSET SET CONTEXT context-name [/DATABASE=database-file]

       DECSET SET DATABASE database-file

  DCL wild cards may not be used in either the context-name or
  database-file argument.

  The /DATABASE qualifier allows you to set a context that is defined
  in the specified database.  If the /DATABASE qualifier is missing,
  the value of the DECSET$CONTEXT_DB logical name is used; if this
  logical name is undefined, your SYS$LOGIN directory is searched for
  a file named DECSET$KNOWN_CONTEXTS.DECSET_CONTEXT_DB.  If a database
  cannot be located, an error message is displayed.

  The DECSET SET CONTEXT command automatically defines the logical names
  DECSET$CONTEXT and DECSET$CONTEXT_DB.  The DECSET SET DATABASE command
  automatically defines the logical name DECSET$CONTEXT_DB.