Luse maybe I can help you understand the core concept.

A baby is born. mentally and physically that baby is one whole person.
At age two that same whole child gets abused by someone they know.
this child has to be around this abuser.
physical pains heal but mental pain doesnt always heal right away.

imagine having to be around / near / talk to / be nice to / mind your manners with /respect someone who has hit you, raped you, pulled your hair, and other horrors.

its sometimes easy for adults to bite their tongue or walk away from those that do wrong to them.

but this 2 yr old child has been taught you dont say no, you mind your elders, dont walk away when Im talking to you, dont be sassy...

now the child is in mental stress. this child is being physically and mentally abused and cannot get away from the abuser and cannot tell anyone whats going on either.

so the child dissociates. they mentally separate a part of their self. the side they dissociate away from their self is an alter and the rest is the core.

Another word for the "core" is called the "host" the person in which the alters live inside of.

As time goes on and more physical and mental abuse happens. the "core" (host) splits off more parts of their self.

the parts of their self that is split off is called the alters and the person in which all the alters live in is called the "core" or in some places called "the host"

sometimes the host (other wise called the core) dissociates so much that it is the alters that are taking care of every day life, and the host (core) dissociates their self into a daydream world where they feel safe.

now you have my alters Sunny, Rainy, Opal, Thistle, Red, Yellow and Green getting up in the morning, making breakfast, getting the family off to school and work, going to work, paying the bills, taking care of the children, watching tv,....

and the host (otherwise called the "core") is off in their mental dream world hidden from the outside the body world.

if the host (core) is hidden like this it can take yrs before any communication between the alters and the host can take place.

here on psych central we have many different people from many different locations from around the world.

some places use the term "core" to describe the person in which the alters live. Some places use the term "host" to describe the person in which the alters live. and some places like here in NY use the two terms interchangeably.

Everyone here on psych central is at different places in the healing too.

some people are in the denial stage.
some people are in the establish communication stage
some people are in the working together as a team stage
some people are in the discovering what their alters parts and their host(core) parts system is
and some people are in the integration stage.

the "core concept" is the same as your "parts of a whole concept". its just that people are using the wording from their own locations and from where they are along the healing path.