In relation to alters fragments is just a word to describe alters that are very restricted in function or experience. Like some alters function a lot in the world and can deal with a whole range of aspects of life so they can seem "fuller". Others might hold a single aspect of a traumatic memory and not be required again for anything else. Because of that they may not develop further as an alter and not accrue more life experience. I have a few fragmented alters like that. A "fragmented alter" has never been a diagnostic word in relation to DID, its just a descriptive word used to describe an alter or part that has a very specific and limited function.

And yes, what you describe is a normal way of coping with / remembering trauma, whether you have DID or not.