What does the Client Access tab do under the user Settings?
This will allow the Admin to grant the user access to all clients or only a few clients in their account.
Full Access: this means this user has full access and can add time towards all clients in the account.
Limited Access: this means you can limit the user to only a few clients you want them to be able to access and add time for.
Note* This type of access can also be dictated in the clients "User Access" tab when adding clients into the system as well.