Generally speaking, an FMO is the same thing as an IMO, but on the annuity side of the business as opposed to the life insurance side.
With more life insurance groups growing their annuity business and vice versa, the lines between IMO and FMO have become blurred, leading to some confusion. It might be easiest to just regard “IMO” and “FMO” as interchangeable terms with no real difference between them, other than perhaps providing some insight into what products they mainly specialize in.