anchor: Characteristics related to useful life are that  has an indeterminate or an unpredictable useful life because of the unusual manner in which it is used, improved, retired, modified, or maintained or is at a very high risk of being destroyed during use or of premature obsolescence.
positive: The length of a useful life depends on how it is used, modified or maintained and unpredictable use can lead to a higher risk of it being destroyed prematurely.
negative: The manner in which an owner uses a product or takes care of it will not impact the length of its useful life.