ok I'm genuinely confused what these are and when you are supposed to send them.

I know you can send updates to school but are those like the letter of interests? I don't have any real updates like no new pubs or anything so didn't think about sending one.

So am I supposed to send these letter of interests to schools I haven't gotten an interview yet but have strong interest and letter of intent to the schools I already interviewed at? sorry I didn't think about these but saw so many posts here, reddit, and tiktok....