This is a good introduction to IFS sub-personalities I think .....
"Richard Schwartz, who formulated IFS, began his career as a family systems social worker. Schwartz discovered that, in troubled families, individuals were often trapped in unconscious patterns of behaviour that caused conflict and unless these patterns could be made conscious, family dynamics were not likely to make significant changes. As his focus shifted to the client’s report of inner experience, Schwartz began to recognize that just as a family has individual members with different roles so too the individual psyche is comprised of what his clients referred to as “Parts”. A part, he began to realize, is not just a temporary emotional state of habitual thought patterns; rather, it is a discrete and autonomous system that functions with a particular role– as a sort of ‘subpersonality‘ . He learned about the autonomy of each part through experiences with his clients and his own Parts. While we are all made up of many parts, there is a central part or capacity which Schwartz calls the “Self”; other disciplines have noticed and referred to the Self as “witness consciousness” or “aware ego” or “observing ego”. Qualitatively, the Self, in Schwartz’s understanding, is different from the parts in that it is designed to be the natural leader of the psyche."
"Schwartz wove different approaches into his knowledge of family systems to formulate IFS therapy which he describes as collaborative, non-pathologizing and accepting. Rather than trying to get rid of our less desirable or difficult qualities, all parts of our personality are considered valuable."
https://majohnston.wordpress.com/working-with-our-inner-parts/
I have made notes in blue to indicate where the teachings in the quote below deviate from Dr Schwartz' original, and in my mind, better understanding of the value and ongoing purpose of sub-personalities. I hope it's not too confusing.
When children can't get their needs met by being themselves, they will attempt to adjust who they are. By acting in different ways, they can develop mask-like personalities that will carry on into adulthood. In contrast, an emotionally healthy person does not have a "personality" protecting their real self, they just are who they are. Unfortunately, most of us had to create one main personality and a host of other subpersonalities. These characters were useful in order to survive our neurotic families, but now that we are adults, their existence can cause a host of problems in our relationships and careers.
Most people don't use the word "subpersonalities" but are very aware of the moods, eccentricities, acts, or special "ways" of our family members, friends, and co-workers. How many times have you said or heard statements such as:
"I hate it when he acts like such a know-it-all!"
"There she goes again, acting the clown!"
"Here comes Mr. Encyclopedia."
"That sweet-little-girl act drives me crazy!"
Sometimes we will consciously put on an act to meet a certain situation, but most of the time we are unaware of the false selves ** (Dr Schwartz, doesn't refer to sub-personalities this way - rather as "parts" of ourselves ....) we have created. Subpersonalities literally become "second nature." They rise and fall in response to various stresses and triggers in our lives. Unfortunately, "second nature" is just another phrase for "unconscious." That which remains unconscious will continue to affect our lives in unpredictable ways.
Healing is about awareness, feeling, and integration, and the first step is recognition. In the case of subpersonalities, recognition often begins with the process of identifying your different false selves ** (as above). Although this may be intimidating and unnerving, it will help you understand aspects of your behaviour that have remained a mystery.
It is often easier to recognize the subpersonalities in others first. When we can do that, our own will become more obvious. To assist in this matter, I have compiled a list of subpersonalities offered by some colleagues of mine. You may find parts of yourself here!

Even though the variations are limitless, you may notice from this list certain commonalities that appear in slightly different ways. Since children have common needs, and dysfunctional families have common forms, we often share similar subpersonality types.
If you are interested in identifying your subpersonalities, start by writing down names for your different moods, acts, or behaviours. It may be useful to do the exercise with supportive, non-critical friend or family member, since you may see subpersonalities in each other that are difficult to notice individually.
When you are finished your list, you may have as few as the "classic triangle" (the inner parent, the adult, and the inner child) or you may have more than ten! If you have a large number, look to see if there are any parts that are linked in some way and may actually be variations of one subpersonality. For instance, "Mr. Know-it-all" may be linked with "The Saint" and actually be "Mr. Good and Right."
If your list seems complete, look to see if there are opposing pairs, such as:
The Devil - The Saint
Sexy - Sexless
The Brain - Mr. Confused
Ms. Fearless - Scared Little Girl
Mr. Good Guy - The Judge
The existence of these types of pairs will indicate certain dynamics that may operate in your life like a swinging pendulum. They are often the main characters in situations of inner conflict.
Once you recognize your subpersonalities, the next step is to observe them in action. You may notice situations that trigger their appearance, and discover what their purpose is **. If you enter your observations in a daily journal, you may find that they relate to events and situations in your childhood. It is often helpful to remember that all subpersonalities, no matter how troublesome, arise out of a genuine need.** Subpersonalities are an attempt to protect, get needs met, and make life better. They helped us survive and need to be respected ** - even if they have overstayed their welcome. ** (Dr Schwartz says "have their roles adjusted" instead, and maintains that our sub-personalities are a legitimate part of our personality make up)
The Royal "We" takes on a deeper meaning!
http://www.primalworks.com/thoughts/thought030714.html