The Permission Preview feature is available to full Admin (Administrator) users only and allows you to view requests and the application as a requester or another staff person. This will give you a better understanding of visibility within a request as well as the portal in general.
Important Note:
You can take actions while you are previewing permissions. However, any action you take will be associated with your user account, not the user you are previewing.
Use Permissions Preview
Hover over the Profile icon in the upper right-hand corner, then select the Permission preview option:

Select a user to impersonate from the dropdown menu or type their name or email address to search:

Notes:
There are also generic users you can select to view the portal at that permission level in general.
Impersonation allows you to preview what the application looks like as this user, including which controls and content they can access. You can only select other users who have lower-level permission roles in the application. Any action taken during the preview will still be attributed to your user account.
Click the Set Permissions button:

A “Permission view on” banner will show which user you are viewing the portal as; navigate the portal as usual to see how various pages look to that user:

When you are finished, click the Turn off permissions preview button to exit the preview:

Common Errors
In some cases, you may encounter an error when you attempt to navigate to a page that the user does not have access to. For example, if you try to preview a request as a requester user who did not submit the request, you may receive a “The user you have selected does not have access to this page“ error if that user does not have access.

In other cases, if you begin the permission preview session on a page that the user doesn't have access to. For example, if you use permission preview to view as a requester while on the Admin dashboard, you'll receive an “Oops! Something went wrong“ error and be brought to the front page of the portal.

Both errors indicate that the user you are attempting to preview does not have access to that section or page in some way.
Taking Action on a Request in the Preview
While you are previewing permissions, you will see the actions that are available to that user. However, if you take an action, like sending an external message, the action will be recorded in the timeline as being associated with your user account. You cannot impersonate a user and take actions as that user; you are only previewing what the portal looks like to them.