The F trim is a flat flashing, mainly used at the intersection of a pitched roof and flat roofs often found on dormers. The F trim should not be laminated over completely as it will crack. It is nailed or stapled to the deck and bent up the roof slope. In this situation.
The F trim also acts as an expansion facility and must only be fixed to the deck along the bottom edge. There are many other applications for F trim including vertical details where laminating would be time consuming, under the feet of air conditioning units to enable re-roofing without disconnecting, and use on some parapet wall details etc.
The trim should be nailed to the deck around its edges and any joins or nail penetrations should be bandaged over. Any unlaminated trim can be topcoated with the rest of the roof.