Who writes it
The letter of invitation is the signed letter from the family member in Canada inviting a parent or grandparent to visit on a super visa. It is written by the host, not the applicant: IRCC describes it as “Your letter of invitation, written and signed by your host child or grandchild, must include” the required details.
What it must contain
Those details are specific — the letter must show “proof that they meet or exceed the minimum necessary income,” list the people counted in the host’s family size, and, IRCC adds, “The letter must also include the name and date of birth of each person.”
In other words, the invitation doubles as the host’s financial-support declaration, which is why an incomplete letter is one of the most common reasons a super visa file is returned. Treat it as a checklist, not a note.