F.I. is a Field Investigation of an individual, kind of like a OHP contact report. It's a way for TPD to keep track of persons of interest. TPD records of high profile people contain a list of know friends, F.I's contribute to these lists when used.
I heard them the other day talking about a guy, saying they needed a picture of him to put on their "Dirty Thirty" list. I bet those are some great individuals.