What are Banknotes and how can you get them?
  Banknotes are a premium currency in the game. Unlike energy, they do not regenerate automatically, but you can obtain them in the following ways:

• Purchase with real money;

• Earn as a reward for participating in limited-time events;

• Find them in chests, treasure troves, and gifts on the expedition map;

• Receive them from merging items on the merge field;

What can Banknotes be spent on:

• Buying merge energy and expedition energy;

• Buying resources needed to complete expedition tasks (instead of collecting them manually);

• Opening bubbles with items on the merge field;

• Expanding the backpack (increasing inventory slots on the merge field).