When you create a new
diary group, or visit one, gnus-diary 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).