The Enneagram personality types (enneatypes) evolved into their present form through the work of a psychiatrist named Claudio Naranjo during the 1970s. As others began to teach and write about the enneatypes, additional concepts and interpretations were introduced into the system. So, although there is agreement among all teachers that there are nine personality types in the system, there is not always consistency as to what defines each type. Much of this inconsistency stems from attempts to define type by describing personality itself.
The label of Personality Type can be very misleading. Personality is unique to an individual. Type is what's common to a group or category of people. What's common between people of a certain Enneagram type isn't personality. It's something underneath personality.
Two people of the same Enneagram type share a common focus for how they organize their reality and experience. This in turn has a major influence upon how they approach life, which gets expressed uniquely through their personality. Because personality is unique for each individual, no two people will express Enneagram type in exactly the same way. While there are many influences upon personality for an individual, Enneagram type is distinct in its influence because it organizes the other influences through its focus.
As an example, if type 1 is focused on acceptability through behavioral correctness then what is acceptable or correct for an individual comes from that individual's unique set of life influences and resulting standards. So, although two people of the same Enneagram type (type 1 for example) will organize their lives around a similar focus (acceptability and correctness), what goes into that for each individual is unique (individually determined standards for acceptability and correctness). Subsequently, what comes out of that as expressed through personality will also be unique to each individual (what needs to be corrected and how).
The book Personality and Ego: Exploring the Core Focus of the Enneagram Types delves deeper into the experience underlying or driving personality type. It does this for each type by describing the core focus (the goal and the means), the attentional habit, and how those in turn might influence personality.