The initials BMF can mean either bare metal finish or Bare Metal Foil. The latter is a product name- a metal foil with adhesive on back. the term bare metal finish refers to any and all techniques to duplicate a natural (unpainted) metal- usually aluminum- on model subjects.
Some modelers do a great job with paints, aluminum, silver, matt or glossy. Sometimes they mix slight variations in colors for multiple passes to simulate differences in alloys or surface smoothness.
There are metalizers, like Alclad or others, that are very thin metallic paints that go over a glossy undercoat and can replicate surfaces from weathered aluminum to polished aluminum or chrome plating.
Finally, there are some who do whole aircraft with Bare Metal Foil. A lot of work, but can result in a striking model.