Any newly arrived camper is given the rank of a probatio and a tablet with the word on it. Probatio is considered the lowest rank at camp as it is only given to people that haven't been claimed at Camp Jupiter or have not accomplished a worthwhile deed. Once somebody loses their probatio tablet, he or she becomes an official member of the legion.
Frank Zhang was still at this rank after a few weeks at camp until Percy Jackson arrived. He was charged with cleaning the armory and taking care of Hannibal, the elephant, tasks considered beneath the other campers. Percy was also given this rank when Octavian accepted him into the Legion.
Later, when deciding a cohort to place Percy in, Frank speaks up for placing him in the Fifth Cohort, but because he is only a probatio, he isn't allowed to represent Percy. Hazel Levesque, who is now a legionnaire, steps in and says that she will represent Percy instead. Frank didn't become a centurion until Mars appeared and told him to lead a quest to Alaska. Since only centurions can lead a quest, Reyna Avila Ramírez-Arellano was forced to make Frank a centurion.
Five days after arriving at Camp Jupiter, Percy becomes a Praetor from a probatio without being a legionnaire or centurion after helping defend Camp Jupiter against the Giant Army and defeating the giant Polybotes.
A female probatio appears in the Barnes & Noble edition of the book, looking into an ancient Roman mystery.
- Probatio means "test" in Latin, referring to how those on probatia must prove themselves to the Legion before becoming members.