The rule of 90 is referring to the appropriate number of certain things you'd need if a civil or worldly crisis were to occur and you'd be put into a survival situation.
For instance if a zombie apocalypse were to occur through the rule of 90 you'd need food to last atleast 90 days, a fire arm with atleast 90 rounds of ammunition, water to last 90 days and a map which shows atleast 90 miles squared of the surrounding area.
Again if you survived a plain crash through the rule of 90 you'd need to find supplies to last 90 days, you'd wait by the crash sight for 90 days to be rescued, when setting up a shelter you'd set it no more than 90,000 metres away from the crash sight.
The changing of gameboard rules like adding another game board piece that doesn't normally belong there to create I different game with a different experience.
When my friend and I get board of the same board game we try to rule swap it with another board game to have more fun!