[quote=@LeiaHair]
[@redxiii]......... It can be all three. 

Does that clear it up? 
[/quote]

Yes.  Ultimately character deaths are imposed upon, but you can present added hypotheticals for your character to create ways to escape impending death and as long as those hypotheticals are deemed worthy, your character will live to see another day, but said worthiness will be determined by the GM, thereby falling under the imposed category.  Under heated dispute, dice will get involved and ultimately rule, which would also fall under the Imposed category.  Deaths can be contested based on writing skill (Can you write a more clever escape versus the other player's clever attack, as judged by the GM) but are, for the most part, imposed upon by someone (or something) other than the character's creator.

There does not exist a rule or policy to which a player can say "I do not want my character killed for the entirety of the RP" and therefore character deaths are not agreed upon, but players are encouraged to try their best.