FYI stands for for your information. You put it on short business notes and emails when you are telling someone something they need to know — a time change, a file, a fact — and they usually do not have to do a job with it, and they usually do not have to reply. It can sit in the subject line alone, or at the start of a sentence: FYI, the meeting moved. It is work language, quick and a little dry. Though actually, with a friend it can feel sharp, as if you are throwing paper on their desk with no hello. At work that dryness is normal. It is not a request, and it is not an apology; it is a flag that says please know this.