A death certificate or Certificate of Death was an official report that documented the death of an individual. They could be invaluable sources of information when performing genealogical research. While researching her ancestor Shannon O'Donnell, Kathryn Janeway preferred such "raw material" sources to various histories of 21st centuryEarth, as she found them to all be biased. (VOY: "11:59")
The death certificate viewed in "The Next Phase" was a modification of the personnel file viewed in "Conundrum". When Crusher certified Ro's death in the personnel file, she changed her Current Status to "Deceased" and added the date of death.