Lin Xu placed the dried black plum, dried hawthorn, licorice, and aged tangerine peel into clear water to soak. These ingredients, being dried goods, couldn't be cooked immediately. Instead, they had to be soaked and rehydrated first, as only then could their flavors and medicinal properties be fully extracted during cooking.
As for the Dried osmanthus, it wasn't needed for soaking. This ingredient doesn't withstand boiling. It could only be added after the Sour Plum Soup was finished and the heat turned off, using the residual heat in the pot to infuse the soup with its fragrance. If it were boiled, the osmanthus fragrance would dissipate with the steam, and its aroma would actually become milder.
