During a dive, Suunto D5 calculates partial pressure of oxygen (pO2), central nervous system toxicity (CNS%) and pulmonary oxygen toxicity, tracked by OTU (oxygen toxicity units). The oxygen calculations are based on currently accepted exposure time limit tables and principles.
By default in Air/Nitrox dive mode, CNS% and OTU values are not displayed until they reach 80% of their recommended limits. When either value reaches 80%, Suunto D5 notifies you and the value stays in the view.
You can customize views to always show CNS% and OTU. See How to customize dive modes with DM5. See also the Suunto Support for Q and A on DM5 at https://www.suunto.com/Support/software-support/dm5/dm5-faq/