What is meaning of Floodable length in passenger ships as per SOLAS?

As per SOLAS chapter II-1 , regulation 4,

Floodable length is  the amount the ship can be flooded without the risk of the ship sinking. The floodable length is the maximum length of the ship that can be flooded without submerging the margin line, also known as the deck line. The margin line is a line just below the top of the bulkhead deck, or the highest deck which has watertight bulkheads forming watertight compartments. If the margin line is submerged, the ship is serious danger of sinking from flooding.

This can result in a dangerous situation called down flooding where watertight compartments are filled from the top causing a loss of buoyancy and the ship to sink.

Each ship has a unique floodable length and most are designed such that the spacing of bulkheads will allow two adjacent watertight compartments to be flooded without submerging the margin line and endangering the ship.

Points to pounder for floodable length in passenger ship
• The floodable length at any point of the length of a ship shall be determined by a method of calculation which takes into consideration the form, draught and other characteristics of the ship in question.
• In a ship with a continuous bulkhead deck, the floodable length at a given point is the maximum portion of the length of the ship, having its centre at the point in question, which can be flooded under the definite assumptions set forth in regulation without the ship being submerged beyond the margin line.
• In the case of a ship not having a continuous bulkhead deck, the floodable length at any point may be determined to an assumed continuous margin line which at no point is less than 76 mm below the top of the deck (at side) to which the bulkheads concerned and the shell are carried watertight.
• Where a portion of an assumed margin line is appreciably below the deck to which bulkheads are carried, the Administration may permit a limited relaxation in the water tightness of those portions of the bulkheads which are above the margin line and immediately under the higher deck.
Note:
1. In determining the floodable length, a uniform average permeability shall be used throughout the whole length of each of the following portions of the ship below the margin line:
• The machinery space as defined in regulation ;
• The portion forward of the machinery space; and
• The portion abaft the machinery space.
2. The uniform average permeability throughout the machinery space shall be determined from the formula:

85 + 10( a-c)/v

where:
a =  the volume of the passenger spaces, as defined in regulation 2, which are situated below the margin line within the limits of the machinery space;
c = the volume of between-deck spaces below the margin line within the limits of the machinery space which are appropriated to cargo, coal or stores;
v = the whole volume of the machinery space below the margin line.