FL does the traditional floodable length calculations, based on symmetrical flooding and sinkage to a margin line or to a condition of minimum GM. It presents floodable length data at regular intervals along the length of the vessel, including not only the length of flooding but also the trim, sinkage, margin, and GM.
This is a preliminary-design tool, since it does not use any actual compartment boundaries which may be in the vessel model, but rather "designs" its own simple compartments which are as large as they
can be to satisfy the margin line and GM requirements.
Given the ease of modeling and modifying compartments using Part Maker, it is quite possible to do a better job of preliminary design using damage stability (with GHS), especially when the assumptions of uniform symmetrical flooding and the simplistic stability criteria of FL are not valid. But floodable lengths are still useful ... and they are sometimes required.