Final answer:
When a large bank contracts with a custodial service company, they are hiring an external third-party service to handle janitorial and light maintenance duties.
Step-by-step explanation:
When a large bank contracts with a custodial service company to provide janitorial and light maintenance duties for them, they are hiring an external third-party service to handle these tasks. This arrangement allows the bank to focus on its core banking functions while delegating the responsibility of cleaning and maintenance to a specialized company.