Several posts have mentioned that if you have a multiple entry visa then you are REQUIRED to leave the country every ninety days. Is this fact or a misunderstanding? I thought the ME visa was only PERMISSION to leave and re-enter, rather than a requirement to leave. On a related note, once you buy a re-entry permit are you then REQUIRED to leave or can you stay if your plans change?

• A ME Non 'O' visa only gives permission to stay in the country for a maximum of 90 days every time you enter. So you are required to leave on or before the 90th day.
• You can enter as often as you want. 
• You are not obliged to use a re-entry permit once you've bought it.
• You don't need a re-entry permit with a ME Non 'O' unless you want to keep the last entry/permit to stay alive after the visa has expired.

A multiple entry non-o visa from an embassy or consulate only allows a 90 day entry. You must leave and re-enter the country to get a new entry at the end of the 90 days.

A re-entry permit is obtained for a permit to stay. The dates you put on the application form for one mean nothing since they are not shown on the re-entry permit stamp.