When you create a new diary group, or visit one,
automatically checks your group parameters and if needed, sets the
summary line format to the diary-specific value, installs the
diary-specific sorting functions, and also adds the different
X-Diary-* headers to the group's posting-style. It is then easier
to send a diary message, because if you use C-u a or C-u m
on a diary group to prepare a message, these headers will be inserted
automatically (although not filled with proper values yet).