"In my claims I see an option to write off the amount outstanding. What is this?"

A write off should only be done when you are eligible for the claim but due to circumstances such as invalid patient insurance, you cannot recover payment. This would qualify as a true write off that you can use for tax purposes. 

Any other claims that are just refused because they do not meet the MSP criteria should be cancelled instead as they do not qualify as a true tax write-off.