The Tritype concept evolved from Oscar Ichazo's trifix concept. Katherine Chernick Fauvre coined and trademarked the term Tritype®. It basically looks at an individual in terms of three types. An individual identifies the dominant type from each center (i.e., one type from the gut center: 891, one type from the heart center: 234, and one type from the head center: 567). A preferential order to those three types is also identified.
For example, an individual may identify type 1 as the gut type, type 3 as the heart type, and type 5 as the head type. Of those three types, the preferential order may be 5-1-3.
Additional forms of Tritype are sometimes used as well such as Tritype Archetype which disregards the preferential order and Tritype with Wings which adds the preferred wing to each of the three types.
The book Quick Guide to the Enneagram explains the various forms of the Tritype concept. The 2nd book in the Enneagram User Guide series, Personality Variations, briefly describes the trifix concept and then explains the various forms of the Tritype concept along with how the core focus can be used with it.