These are basic summaries of what we expect to see in the backstory of your characters. Unless it is directly stated as being optional such as time alive for shinigami, hollows, and vizards, then it is a required part of the application.
Keep in mind that it is important to keep these parts balanced, we don't want to read 5 paragraphs on time in rukongai and two sentences about becoming a captain.
All backstories should be between 3 and 7 paragraphs. If you exceed seven paragraphs then it is the right of the staff to skip any optional pieces of your backstory. If there is no optional piece for us to skip then you may be asked to shorten the backstory. Remember this is a summary of existence, not a detailed novel.
For Shinigami -
Events of life culminating in your death. (Optional due to lack of memory when dead.)
Life in Rukongai.
Entrance into Soul Society and the Shinigami Academy.
Progression up the ranks.