org-agenda-diary-file

org-agenda-diary-file is a variable defined in `org.el'.
Its value is

diary-file

Documentation:
File to which to add new entries with the `i' key in agenda and calendar.
When this is the symbol `diary-file', the functionality in the Emacs
calendar will be used to add entries to the `diary-file'. But when this
points to a file, `org-agenda-diary-entry' will be used instead.

You can customize this variable.