From the introduction:
The Intermediary Class is a new concept for the D20 Modern ruleset. A starting character can choose an Intermediary Class rather than one of the attribute based basic classes, gaining cool abilities without worrying about a Level Adjustment. Intermediary classes are slightly more powerful than basic classes, and slightly weaker than a full advanced class. These unusual classes have special abilities which are balanced by unique defects and weaknesses. Like Templates, intermediary classes straddle the line between risk and reward. Players should think carefully before choosing one of these intermediary classes.
Intermediary classes offer the player slightly less freedom to multiclass than the basic classes. Each intermediary class is ‘keyed’ to an advanced class, representing a period of intense training and apprenticeship. Once a character takes a level of an intermediary class, she is put on the fast track to that advanced class, gaining the abilities necessary to meet that classes entry requirements in a fraction of the usual time.