|d20 Modern System Reference Document|
An advanced class represents a focus and a calling for the experienced adventurer. It provides a specialization and a range of power and ability to give a character that something extra to set him or her apart.
Although each advanced class naturally builds from a certain basic class, every advanced class is available to all characters who fulfill the prerequisites of the class, regardless of what basic classes they have gained levels in. The associations between basic classes and advanced classes are summarized on the following table.
The Gamemaster may add advanced classes specifically suited to his or her campaign. Conversely, the GM can decide that certain advanced classes aren't available in the campaign. Check with your GM before selecting an advanced class.
Qualifying for an Advanced Class
Advanced classes are like basic classes, except that they have requirements that must be met before a character can attain 1st level in the class. A character who qualifies can choose an advanced class as an additional class as he or she gains levels, using the multiclassing rules. Some combination of base attack bonus, feats, and skill ranks determines whether a character is eligible to gain a level in an advanced class.