Spiritual development within scientology is accomplished by studying Scientology materials and/or by receiving auditing. Scientology materials related to training auditors and the auditing processes to be received are structured in sequential levels. For those practicing Scientology within The CoS, these levels are precisely enumerated and laid out in a document called "The Bridge to Total Freedom". This document is subtitled, "The Scientology Classification, Gradation and Awareness Chart of Levels and Certificates" and Scientoligists tend to simply call it "the Bridge". 
The Bridge is laid out in stair step fashion with each step having a name, the sought end result and some other information as well. For instance, one step is called "Grade 0" and the end phenomenon of this step is stated as, "Ability to communicate freely with anyone on any subject." The steps occur in a definite order with a few short alternate pathways in places, but in general, simpler steps occur earlier on the Bridge and more complex ones occur later on. Completing these steps in succession is referred to by Scientologists as "going up the Bridge. The process bears some similarity to receiving successive colored belts as one advances in ability in a martial art; but while both Scientologists and martial arts students receive certificates of completion as each significant level is attained, Scientologists don't get the nifty colored belts. They sometimes will wear a small piece of jewelery though, signifying completion/ attainment of a given training or processing level.
The Bridge has two sides: one labeled 'Training' and the other labelled "Processing". Training means Auditor Training. Along this path the Scientologist is trained in how to perform the specialized spiritual counselling called Scientology auditing. He takes courses and at higher levels completes required internships. The highest level of training is "Class XII, Gold Seal". The Processing path delineates which auditing processes should next be completed upon the Scientologist by a certified auditor."
At any given time a person is said to have both a "Training level" and a "Processing level". They are different and do not proceed in lockstep with one another.
The Church of Scientology believes in the principle of reciprocity, involving give-and-take in every human transaction. Scientologists refer to the giving of something of value in exchange for something of equivalent value as "being in exchange". It is very important to remain in exchange. Accordingly, members are required to either make donations for training and processing as they move up the Bridge or to participate in a "co-audit". A Co-audit involves practicing auditing on a "twin" (A fellow student studying the same materials at the same time). Each twin takes turns auditing the other and in this way overall donation amounts are reduced as the students do not require donations of one another for the auditing they receive.
The donation amounts for both auditor training and auditing generally increase as higher levels are reached.
- ↑ 1.0 1.1 1.2 Scientology Large Classification Gradation and Awareness Chart. Church of Scientology.