According to my database logs it isn't sent for at least the following transaction types (and note that I haven't tested them all, although you can conclude I've tested most of the subcription-related ones):
- send_money (there having been no opportunity for you to send it)
It definitely is sent with the following:
- invoice_payment (although I have them both with and without: this seems to be the 'notes to yourself' field when sending a PP invoice)
- subscr_failed (ditto) (I wasn't always sending it)
- subscr_payment (ditto) (I wasn't always sending it)
- recurring_payment_suspended_due_to_max_failed_payment (ditto) (I wasn't always sending it)
- subscr_signup
- subscr_cancel
- subscr_eot