Probatio is a position given to the lowest and newest ranking legion members of Camp Jupiter.
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 by their godly parent at Camp Jupiter or have not accomplished a worthwhile deed. Once somebody becomes an official member of the legion, they lose their probatio tablet.
Frank Zhang was still at the rank probatio after a few weeks at camp until Percy Jackson arrived. He was charged with cleaning the armory and taking care of Hannibal, the war elephant, tasks looked upon as insignificant by higher ranked 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. Since only centurions can lead a quest, Reyna 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 intimidating giant Polybotes.
A female probatio appears in the Barnes & Noble edition of the book, looking into an ancient Roman mystery.
The story tells of the early days at Camp Jupiter for Claudia, a legacy of Mercury who has recently became a probatio in the Fourth Cohort. Other probatios shown are Blaise, a son of Vulcan, and Janice, a daughter of Janus.
- Probatio means "test" in Latin, referring to how those on probatia must prove themselves to the Legion before becoming members.