Thank you very much for your response. I'm planning to put there the names of my family members who met my husband in person I can still remember the dates of their meeting in person since it was recent. How did you write your explanation? Was it in a letter form with Dear Sir/Madam as salutation?
I think there's a box right under that question that's for any explanations related to it so I put that explanation there.
My big letter was related to the question that asks if there's anything else you'd like to add to support your relationship. I first indicated who was writing it (written by Principal Applicant First name Last name) and then wrote in the box til I ran out of space. Then I continued it on a separate sheet of paper (I indicated where it would be continued) and paper clipped my paper to the back of that form. So I didn't have a salutation since the beginning is just me answering a question from the form.
But a lot of people write these letters in different ways and I'm sure a lot of them do start with Dear Sir/Madam and the like. No need to stress over the details like that - whatever you decide is going to be fine in the end. They get thousands of apps every year and everyone has their own way of filling out stuff, explaining, writing letters, etc.