When Klara experiences a situation as ambivalent or otherwise has trouble analysing what she sees, her visual input becomes "partitioned", divided into separate boxes. This is presumably a reference to the way image recognition software draws boxes around objects that have been identified. The partitioning is never explained, and Klara does not see it as odd, although she does appear relieved when the partitions disappear.
The embrace continued, and when I glanced again at the Mother, that whole section of the room had become partitioned, her narrowed eyes repeated in box after box, and in some boxes the eyes were watching Josie and the Father, while in others they were watching me. (page 186)