Optional classes are classes which come from specific series, or follow a specific specialization that makes them slightly different from one of the base classes. Examples of classes from other series include Jedi from Star Wars or Technomancers from Shadowrun. Specialized classes include the Monk.
GMs are encouraged to ban, allow, or restrict optional classes that they feel wouldn't work with the setting they're using. Some optional classes are similar to each other or similar to base classes.
Factotums can take talents from classes the GM allows them to from the Optional Classes list. If the GM wishes, Factotums can choose talents from banned classes so long as the talents are allowed for the Factotum specifically.