The pockets of Lily’s jacket symbolize her growing maturity.
In Jonas’s community, the children all age at the same time in a special ceremony in December. This is all part of maintaining Sameness. There is a ceremony for each age group from One to Twelve. At One, the babies get a name and a home. At Twelve, the children are assigned their lifetime job and are no longer considered children. They are adults in training.
The ceremonies in between are smaller milestones. Most of them have to do with growing independence and maturity. For example, at age Nine children get bicycles so that they can venture into the community on their own. The Eights get a smaller indication of trust.
Jonas watched and cheered as Lily marched proudly to the stage, became an Eight and received the identifying jacket that she would wear this year, this one with smaller buttons and, for the first time, pockets, indicating that she was mature enough now to keep track of her own small belongings. (Ch. 6)
Lily likely does not have many belongings, because no one really owns anything in Jonas’s society. Everything is provided for them. Jonas is even chastised for bringing an apple home instead of eating it right away. Still, whatever she does have, she now has a pocket for.
The fact that clothing is the same for everyone, and small designations like who has pockets are so important, is an indicator of the conformity in Jonas’s society. Clothing indicates your age when you are a child and your status later in life. Everything is a ritual. Everything has a meaning and a purpose. It keeps everyone working toward the same goal, and maintains Sameness. Sameness keeps everyone safe, happy, and comfortable because everyone knows what to do and what to expect at all times.