What distinction does Kimia make between the two terms of Sara and Mom in the book, Disoriental?

In order to show the reader the horrific way in which "THE EVENT"(13) as it is referred to in the Sadr family had affected her mother Sara, Kimiâ shares with the reader that she had started to refer to her mother as Mom shortly after her mother's descent into near-catatonia. At this point in the narrative, the reader still has no idea what type of tragedy "THE EVENT" (13) constitutes, but it is referred to often and the consequences associated with it are often quite severe. In Sara's case, the unnamed tragedy has resulted in a complete and total loss of her memory, a development made all the more poignant by the reader's knowledge that the Sadr family had been unable to save any of their personal mementos or photographs as they fled from Iran.