There are two different types of replacers in Expense:

• Proxy- can record expenses for other users.
• Temporary approver - can approve expenses for other approvers.

As a user, you can find the people you are replacing under the Private mode dropdown.

How to set up proxies for other users as an administrator can be found in the article I want to be able to switch to an other employee on our organization. How do I do that setting?

How you as an administrator set up temporary approvers can be read in the article Temporary approvers.