This was described when Beta 1.7f-3 was distributed.  Not sure if it ever
made the release notes. Here is the relevant prose from Eric:
 
An optional "digest  header" text can be inserted between  the end of the
table of contents  and the beginning of the digest  itself. If a LISTSERV
(not CMS)  file by the  name 'listname  DIGEST-H' is found,  its contents
will  be inserted  at this  point  in the  digest. This  file, like  list
welcome messages, can be maintained by the list owner.
 
The instructions and  job skeleton at the end of  indexes can be replaced
(not  added to)  by  shorter,  customized text  from  the file  'listname
INDEX-H'. Again,  if that  file exists it  replaces EVERYTHING  after the
list of messages. If you want  the instructions to remain, just copy them
to  the INDEX-H  file. The  purpose of  this addition  is to  replace the
instructions with  a version better  tailored to your audience  (no point
explaining about REPLY TEXT if your audience is 100% PROFS).