Trill initiates were those of the Trill species who joined an initiate program in order to train themselves for potential joining with a symbiont. The initiate program consisted of an intense series of exams and evaluations, both mental and physical, that were designed to eliminate all but the most suitable and dedicated applicants. Over five thousand candidates qualified for the training each year. On average, only three hundred symbionts, annually, were available. As such, it did not take much for an initiate to be knocked out of consideration. Only the best and brightest were joined. (DS9: "Dax", "Playing God")
Initiates were first evaluated by the Symbiosis Commission, which was responsible for their training. Near the end of the program the initiates were assigned to a field docent, a joined Trill who evaluated their suitability in real-world environments.
Curzon Dax was known to be a particularly harsh field docent and personally eliminated many candidates, including Jadzia. Jadzia went on to become the only initiate ever to be rejected and then later re-accepted for the program. The experience made her determined to be a different type of docent for initiates when she was charged with evaluating Arjin during his initiate period. (DS9: "Playing God")
The strenuous nature of the evaluation could be intimidating for many initiates, and rejection often led to deep disappointment and loss of purpose. Verad was a striking example. After his rejection from the program, which he believed was unwarranted, he was determined to capture a symbiont from a current host and join with it outside the strictures of the Symbiosis Commission. To this end he removed the Dax symbiont from Jadzia Dax and was briefly joined with it in 2370. (DS9: "Invasive Procedures")