The yellow line indicates the agent's dealings with a third party when the agent is acting on behalf of the principal. When the agent deals with the third party, they are representing the principal, not themselves.
An agent can perform a wide range of acts while representing their principal. For example, an agent can make and receive payments, sign documents, purchase and sell goods, purchase or sell land, receive or make deliveries of goods, carry out banking transactions; make representations; or enter into contracts for services.
When carrying out these transactions with third parties as the representative of their principal, the legal rights and duties normally accrue directly to the principal rather than to the agent.